I believe this is very closely related to issue 756238.  Perhaps a duplicate.  Possibly also related to issue 752333.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ce864914c1d10e96ac62a9ec99158ac48de5d07a

commit ce864914c1d10e96ac62a9ec99158ac48de5d07a
Author: Hongchan Choi <hongchan@chromium.org>
Date: Fri Feb 09 22:03:24 2018

Check finished_source_handlers_ for scheduling clean up task

There has been an obvious mistake in handling stoppable source nodes;
the check looks up active_source_nodes_ instead of
finished_source_handlers_. This CL is one-line change to fix it.

This change reduces 8% of CPU load within AudioDestinationHandler::Render()
call. More profiling data can be found:
https://docs.google.com/document/d/13ogSCFiXWCMjQv-D_nnKkEKoZlN8g8N9WUN1kMHJeKg/edit#bookmark=id.wd27wugm4ok9

Bug: 804946
Test: CL passes existing layout tests
Change-Id: I2dcd3809384732a80b6bec207eeb8a7051a66758
Reviewed-on: https://chromium-review.googlesource.com/911935
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535844}
[modify] https://crrev.com/ce864914c1d10e96ac62a9ec99158ac48de5d07a/third_party/WebKit/Source/modules/webaudio/BaseAudioContext.cpp

Issue 756238 has some updated benchmark results showing that we are much better with this CL than before.  But we also have quite a ways to go compared to Firefox nightly on some tests, especially the granular synthesis test.

Re #3:

Do we have any plan/idea on how to do more optimization?

 Issue 819742  has been merged into this issue.

For the benchmarks, I think we just need to select the tests that we think are too slow and profile them and figure out where all the time is being spent.

I do think we can speed things up a bit more (maybe?) if we fixed  issue 819742 .

The  issue 819742  is fixed by the CL in #2. No?

Read the description in  issue 819742 .  If the main thread is slow, we can still end up calling clean up too many times, even with this fix.