Performance regression when loading sources

[juliandescottes]

Performance regression spotted with the 11-7 bundle release.

See screenshot of DAMP perf charts, reload regressed of 10%:

The test case from https://bugzilla.mozilla.org/show_bug.cgi?id=1366693 really highlights the issue. The test page is simply sequentially loading scripts as fast as possible and measuring the time it takes to load all scripts.

On my machine:

• without devtools opened: < 1s
• with debugger opened (version 10-25): 3s (profile)
• with debugger opened (version 11-7): 10s (profile)

The test case is a zip at https://bugzilla.mozilla.org/attachment.cgi?id=8869957

Precise STRs:

• serve the content of the zip e.g. on http://127.0.0.1:8080
• open http://127.0.0.1:8080
• (the page should display "Loaded in X seconds", X hopefully below 1)
• open the debugger (be careful about opening only the debugger and not any other tools as they can impact the perf)
• reload the page
• page will now take several seconds to load

[juliandescottes]

The performance regression comes from 7ad9038 (which improves JSX syntax highlighting).

On the second profile you can see a lot of time is spent in codemirror (even though no source is actually opened in the UI). Looks like we rerender the Editor component more than we used to?

[nyrosmith]

I can't verify by numbers but I think this should no longer be an issue because of #4653 and #4677

[juliandescottes]

@nyrosmith just tested with the latest revision of master and the issue is still there, sorry :/