Terrible lag in recent versions when editing existing long note

[jxn]

[Description]:

Recently I've noticed Notes has become unusably slow when editing an existing note containing about ~1000 "words", mostly markdown headings and unnumbered lists. The delay between entering a single character and having it appear can be as long as 30 seconds, making it essentially unusable. Opening that note can take just as long.

I noticed that I can start a new note without delay problems. I get an unresponsive script alert pointing the finger at ckeditor.js, but the debugger never actually works when I click that button (debugger panel opens up but it's empty)... no code file selected, not line number or anything. Nothing actually debugging.

[Affected versions]:

4.0.0

[Prerequisites]:

Firefox 60.0b14 Linux 64 bit

[Steps to reproduce]:

It's easy to reproduce on my machine. I just open that longer note. If I create a new long note, it doesn't seem to happen quite as bad, but the more list items / headings I add the slower it seems to get.

Simply opening or trying to interact with the note in any way takes forever to respond. Disabling Sync does not help.

[Expected result]:

Speedy responses to my keyboard events

[Actual result]:

Slow as molasses