Fix performance for large pads

[seballot]

Refs #4228

[JohnMcLear]

@seballot is an absolute ninja :D Well done man!

I'm going to merge but things to note:

1. Now seeing a Mac / Safari error:

[safari OS X 10.15 13.1] responsiveness.js

[safari OS X 10.15 13.1]   Responsiveness of Editor

[safari OS X 10.15 13.1]     -> FAILED : Fast response to keypress in pad with large amount of contents Error: Error:

[safari OS X 10.15 13.1]      expected 354 to be below 300 (http://localhost:9001/tests/frontend/lib/expect.js:99)

[safari OS X 10.15 13.1]     (http://localhost:9001/tests/frontend/lib/mocha.js:64)

[safari OS X 10.15 13.1] 

Probably due to sketchy test. But weirdly azlso seeing:

[safari OS X 10.15 13.1] timeslider_numeric_padID.js

[safari OS X 10.15 13.1]   timeslider

[safari OS X 10.15 13.1]     -> FAILED : Makes sure the export URIs are as expected when the padID is numeric Error: 

[safari OS X 10.15 13.1]     Error: expected -1 to not equal -1 (http://localhost:9001/tests/frontend/lib/expect.js:99)

[safari OS X 10.15 13.1]     (http://localhost:9001/tests/frontend/lib/mocha.js:64)

and obviously Firefox 52 is still crashing due to responsiveness.js, so I'm gonna run the test again, see if those OSX errors go and then merge.

[JohnMcLear]

[safari OS X 10.15 13.1] FINISHED - 107 tests passed, 0 tests failed, 20  pending, duration: 7:4

Must have been a glitch in the underpants.

Merging! Thanks @seballot . @joassouza please confirm you get a better UX now :)

[JohnMcLear]

I think fastRun.sh was committed by accident @seballot ?? If you can remove I can merge :)

[JohnMcLear]

Should this be included in this PR?

[seballot]

Hi ! fastRun was not an accident, but I removed it so the pull request is only focused on the performance thing

[joassouza]

Well done @seballot! I've tested here and works amazingly!
Thanks for your work, I really appreciate that
Merci :)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

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