UI performance issues on Chrome with >5k blocks

[cpcallen]

Describe the bug

A user in the Blockly forum reports Blockly being unusable with >5k blocks.

@NeilFraser made an initial investigation and reported:

First finding is that it's lightning quick in Firefox. No issues there. Likewise, Safari is smooth.

Google Chrome on the other hand is problematic. First it's very choppy (but usable). An examination of the console shows that most of the CPU is going to 'rendering', not Blockly's JavaScript. But leave the page alone for a few minutes, and it becomes utterly unusable.

To Reproduce

See discussion in linked forum post above.

Desktop (please complete the following information):

• OS:
• Browser: Chrome
• Version: 9.1.0

[cpcallen]

Possibly related to #5025.

[ewpatton]

We've seen this behavior for quite a while in App Inventor and we're using a (really) old version of Blockly. Firefox performs much better for some reason and it seems as though it is a regression in Chrome. I could probably go back in our community history to find the point at which the performance degraded, but it's been an issue generally for a while.

[cpcallen]

Another report in the forum, noting that poor performance is observed with ~2k blocks (see 3rd message in thread).

[cpcallen]

Another report in the forum, noting poor performance in Chrome (but not Firefox) with as few as ~1.7k blocks in Blockly v9.2.1.