CDC Synchronize big transaction is too slow

[Tammyxia]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

• Pause changfeed.
• Start transaction operation which has 5GB data in TiDB, this process duration time is about half an hour.
• Resume changfeed, observe the big transaction sync status.

2. What did you expect to see?

3. What did you see instead?
   sync duration time is more than 5 hours，
   

4. Versions of the cluster

   • Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

     Release Version: v5.0.1
     
     
     

Edition: Community
Git Commit Hash: 4141f837ad4655a567119db743b7b752a95f5aa2
Git Branch: heads/refs/tags/v5.0.1
UTC Build Time: 2021-04-17 04:28:49

    ```

- TiCDC version (execute `cdc version`):

    ```

Release Version: v5.0.0-master
Git Commit Hash: 6363c48
Git Branch: heads/refs/tags/v5.0.1
UTC Build Time: 2021-04-17 04:40:20

    ```

[Tammyxia]

The workload in upstream is "start transaction; update more then 10 million rows; commit".

[overvenus]

It's more like a feature request, we will add it to TiCDC roadmap, thanks!