diff --git a/packages/SwingSet/src/devices/mailbox.js b/packages/SwingSet/src/devices/mailbox.js
index 72ae9f414d3..2feb9660951 100644
--- a/packages/SwingSet/src/devices/mailbox.js
+++ b/packages/SwingSet/src/devices/mailbox.js
@@ -72,12 +72,36 @@ import Nat from '@agoric/nat';
 // replace it with one that tracks which parts of the state have been
 // modified, to build more efficient Merkle proofs.
 
-export function buildMailboxStateMap() {
-  const state = harden(new Map());
+export function importMailbox(data, inout = {}) {
+  const outbox = new Map();
+  data.outbox.forEach(m => {
+    outbox.set(Nat(m[0]), m[1]);
+  });
+  inout.ack = data.ack;
+  inout.outbox = outbox;
+  return inout;
+}
+
+export function exportMailbox(inout) {
+  const messages = [];
+  inout.outbox.forEach((body, msgnum) => {
+    messages.push([msgnum, body]);
+  });
+  messages.sort((a, b) => a[0] - b[0]);
+  return {
+    ack: inout.ack,
+    outbox: messages,
+  };
+}
 
+export function buildMailboxStateMap(state = harden(new Map())) {
   function getOrCreatePeer(peer) {
     if (!state.has(peer)) {
-      state.set(peer, { outbox: harden(new Map()), inboundAck: 0 });
+      const inout = {
+        outbox: harden(new Map()),
+        ack: 0,
+      };
+      state.set(peer, inout);
     }
     return state.get(peer);
   }
@@ -92,18 +116,17 @@ export function buildMailboxStateMap() {
   }
 
   function setAcknum(peer, msgnum) {
-    getOrCreatePeer(`${peer}`).inboundAck = Nat(msgnum);
+    getOrCreatePeer(`${peer}`).ack = Nat(msgnum);
   }
 
   function exportToData() {
     const data = {};
     state.forEach((inout, peer) => {
-      const messages = [];
-      inout.outbox.forEach((body, msgnum) => {
-        messages.push([msgnum, body]);
-      });
-      messages.sort((a, b) => a[0] - b[0]);
-      data[peer] = { outbox: messages, inboundAck: inout.inboundAck };
+      const exported = exportMailbox(inout);
+      data[peer] = {
+        inboundAck: inout.ack,
+        outbox: exported.outbox,
+      };
     });
     return harden(data);
   }
@@ -115,10 +138,13 @@ export function buildMailboxStateMap() {
     for (const peer of Object.getOwnPropertyNames(data)) {
       const inout = getOrCreatePeer(peer);
       const d = data[peer];
-      d.outbox.forEach(m => {
-        inout.outbox.set(Nat(m[0]), m[1]);
-      });
-      inout.inboundAck = d.inboundAck;
+      importMailbox(
+        {
+          ack: d.inboundAck,
+          outbox: d.outbox,
+        },
+        inout,
+      );
     }
   }
 
diff --git a/packages/cosmic-swingset/lib/ag-solo/chain-cosmos-sdk.js b/packages/cosmic-swingset/lib/ag-solo/chain-cosmos-sdk.js
index 79e93f51063..1a4ba3e10fa 100644
--- a/packages/cosmic-swingset/lib/ag-solo/chain-cosmos-sdk.js
+++ b/packages/cosmic-swingset/lib/ag-solo/chain-cosmos-sdk.js
@@ -256,7 +256,7 @@ export async function connectToChain(
         log(`helper said: ${stdout}`);
         try {
           // Try to parse the stdout.
-          return JSON.parse(JSON.parse(JSON.parse(stdout).value));
+          return JSON.parse(JSON.parse(stdout).value);
         } catch (e) {
           log(`failed to parse output:`, e);
         }
diff --git a/packages/cosmic-swingset/lib/block-manager.js b/packages/cosmic-swingset/lib/block-manager.js
index 31810622475..9f6821a9865 100644
--- a/packages/cosmic-swingset/lib/block-manager.js
+++ b/packages/cosmic-swingset/lib/block-manager.js
@@ -151,8 +151,8 @@ export default function makeBlockManager({
 
         // Always commit all the keeper state live.
         const start = Date.now();
-        const { mailboxSize } = saveChainState();
-        const mbTime = Date.now() - start;
+        saveChainState();
+        const chainTime = Date.now() - start;
 
         // Advance our saved state variables.
         savedActions = currentActions;
@@ -167,7 +167,7 @@ export default function makeBlockManager({
         const saveTime = Date.now() - start2;
 
         log.debug(
-          `wrote SwingSet checkpoint (mailbox=${mailboxSize}), [run=${runTime}ms, mb=${mbTime}ms, save=${saveTime}ms]`,
+          `wrote SwingSet checkpoint [run=${runTime}ms, chainSave=${chainTime}ms, outsideSave=${saveTime}ms]`,
         );
         currentActions = [];
 
diff --git a/packages/cosmic-swingset/lib/chain-main.js b/packages/cosmic-swingset/lib/chain-main.js
index 9738aca5ed5..a2a7bc8d941 100644
--- a/packages/cosmic-swingset/lib/chain-main.js
+++ b/packages/cosmic-swingset/lib/chain-main.js
@@ -1,10 +1,69 @@
 import stringify from '@agoric/swingset-vat/src/kernel/json-stable-stringify';
+import {
+  importMailbox,
+  exportMailbox,
+} from '@agoric/swingset-vat/src/devices/mailbox';
 
 import { launch } from './launch-chain';
 import makeBlockManager from './block-manager';
 
 const AG_COSMOS_INIT = 'AG_COSMOS_INIT';
 
+const makeChainStorage = (call, prefix = '', imp = x => x, exp = x => x) => {
+  let cache = new Map();
+  let changedKeys = new Set();
+  const storage = {
+    has(key) {
+      // It's more efficient just to get the value.
+      const val = storage.get(key);
+      return !!val;
+    },
+    set(key, obj) {
+      if (cache.get(key) !== obj) {
+        cache.set(key, obj);
+        changedKeys.add(key);
+      }
+    },
+    get(key) {
+      if (cache.has(key)) {
+        // Our cache has the value.
+        return cache.get(key);
+      }
+      const retStr = call(stringify({ method: 'get', key: `${prefix}${key}` }));
+      const ret = JSON.parse(retStr);
+      const value = ret && JSON.parse(ret);
+      // console.log(` value=${value}`);
+      const obj = value && imp(value);
+      cache.set(key, obj);
+      // We need to add this in case the caller mutates the state, as in
+      // mailbox.js, which mutates on basically every get.
+      changedKeys.add(key);
+      return obj;
+    },
+    commit() {
+      for (const key of changedKeys.keys()) {
+        const obj = cache.get(key);
+        const value = stringify(exp(obj));
+        call(
+          stringify({
+            method: 'set',
+            key: `${prefix}${key}`,
+            value,
+          }),
+        );
+      }
+      // Reset our state.
+      storage.abort();
+    },
+    abort() {
+      // Just reset our state.
+      cache = new Map();
+      changedKeys = new Set();
+    },
+  };
+  return storage;
+};
+
 export default async function main(progname, args, { path, env, agcc }) {
   const portNums = {};
 
@@ -118,49 +177,13 @@ export default async function main(progname, args, { path, env, agcc }) {
   // so the 'externalStorage' object can close over the single mutable
   // instance, and we update the 'portNums.storage' value each time toSwingSet is called
   async function launchAndInitializeSwingSet() {
-    // this object is used to store the mailbox state. we only ever use
-    // key='mailbox'
-    const mailboxStorage = {
-      has(key) {
-        // x/swingset/storage.go returns "true" or "false"
-        const retStr = chainSend(
-          portNums.storage,
-          stringify({ method: 'has', key }),
-        );
-        const ret = JSON.parse(retStr);
-        if (Boolean(ret) !== ret) {
-          throw new Error(`chainSend(has) returned ${ret} not Boolean`);
-        }
-        return ret;
-      },
-      set(key, value) {
-        if (value !== `${value}`) {
-          throw new Error(
-            `golang storage API only takes string values, not '${JSON.stringify(
-              value,
-            )}'`,
-          );
-        }
-        const encodedValue = stringify(value);
-        chainSend(
-          portNums.storage,
-          stringify({ method: 'set', key, value: encodedValue }),
-        );
-      },
-      get(key) {
-        const retStr = chainSend(
-          portNums.storage,
-          stringify({ method: 'get', key }),
-        );
-        // console.log(`s.get(${key}) retstr=${retstr}`);
-        const encodedValue = JSON.parse(retStr);
-        // console.log(` encodedValue=${encodedValue}`);
-        const value = JSON.parse(encodedValue);
-        // console.log(` value=${value}`);
-        return value;
-      },
-    };
-
+    // this object is used to store the mailbox state.
+    const mailboxStorage = makeChainStorage(
+      msg => chainSend(portNums.storage, msg),
+      'mailbox.',
+      importMailbox,
+      exportMailbox,
+    );
     function doOutboundBridge(dstID, obj) {
       const portNum = portNums[dstID];
       if (portNum === undefined) {
diff --git a/packages/cosmic-swingset/lib/launch-chain.js b/packages/cosmic-swingset/lib/launch-chain.js
index a671b0e5642..7bb071d5b86 100644
--- a/packages/cosmic-swingset/lib/launch-chain.js
+++ b/packages/cosmic-swingset/lib/launch-chain.js
@@ -1,7 +1,6 @@
 import path from 'path';
 import anylogger from 'anylogger';
 
-import djson from 'deterministic-json';
 import {
   buildMailbox,
   buildMailboxStateMap,
@@ -18,7 +17,7 @@ const log = anylogger('launch-chain');
 const SWING_STORE_META_KEY = 'cosmos/meta';
 
 async function buildSwingset(
-  mailboxState,
+  mailboxStorage,
   bridgeOutbound,
   storage,
   vatsDir,
@@ -30,8 +29,7 @@ async function buildSwingset(
   if (config === null) {
     config = loadBasedir(vatsDir);
   }
-  const mbs = buildMailboxStateMap();
-  mbs.populateFromData(mailboxState);
+  const mbs = buildMailboxStateMap(mailboxStorage);
   const timer = buildTimer();
   const mb = buildMailbox(mbs);
   const bd = buildBridge(bridgeOutbound);
@@ -48,7 +46,7 @@ async function buildSwingset(
   await controller.run();
 
   const bridgeInbound = bd.deliverInbound;
-  return { controller, mb, mbs, bridgeInbound, timer };
+  return { controller, mb, bridgeInbound, timer };
 }
 
 export async function launch(
@@ -62,11 +60,6 @@ export async function launch(
 ) {
   log.info('Launching SwingSet kernel');
 
-  log(`checking for saved mailbox state`, mailboxStorage.has('mailbox'));
-  const mailboxState = mailboxStorage.has('mailbox')
-    ? JSON.parse(mailboxStorage.get('mailbox'))
-    : {};
-
   const tempdir = path.resolve(kernelStateDBDir, 'check-lmdb-tempdir');
   const { openSwingStore } = getBestSwingStore(tempdir);
   const { storage, commit } = openSwingStore(kernelStateDBDir);
@@ -76,8 +69,8 @@ export async function launch(
     return doOutboundBridge(dstID, obj);
   }
   log.debug(`buildSwingset`);
-  const { controller, mb, mbs, bridgeInbound, timer } = await buildSwingset(
-    mailboxState,
+  const { controller, mb, bridgeInbound, timer } = await buildSwingset(
+    mailboxStorage,
     bridgeOutbound,
     storage,
     vatsDir,
@@ -86,20 +79,8 @@ export async function launch(
   );
 
   function saveChainState() {
-    // now check mbs
-    const newState = mbs.exportToData();
-    const newData = djson.stringify(newState);
-
     // Save the mailbox state.
-    for (const peer of Object.getOwnPropertyNames(newState)) {
-      const data = {
-        outbox: newState[peer].outbox,
-        ack: newState[peer].inboundAck,
-      };
-      mailboxStorage.set(`mailbox.${peer}`, djson.stringify(data));
-    }
-    mailboxStorage.set('mailbox', newData);
-    return { mailboxSize: newData.length };
+    mailboxStorage.commit();
   }
 
   function saveOutsideState(savedHeight, savedActions) {