Could you copy and past the last 20 or so lines from log.txt? It will be helpful to see what is outputting there all the time.
After copying the lines you can delete log.txt, deleting it might help.
What you pasted looks normal. Are there any quirks of your setup that you can think of? For example, is the note really long? Are there a lot of tags? Do you use a large number of notebooks?
Maybe you can try setting a valid sync directory? Just some random empty folder on your computer will do. I'm thinking that maybe your change in sync method is causing an error when Joplin trys to save changes
I gotta tell you I'm stumped, I think the easiest next step will be to get you to do a full backup (File -> Export All -> Jex) completely wipe your Joplin install. Re-install and restore from backup. I'm fairly confident that will fix this lag. And if you make the backup to jex you won't lose any metadata (I think).
The whole process should only take a few minutes.
OK worked out what was causing the lag. Was playing with the markdown? editor where the lag was non-existent then stumbled upon the problem.
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Spell Checker was the culprit.
That is so odd, Joplin uses the spellchecker that is built in to electron which I would have thought would be very stable.
I'm glad to see you've found a way to make Joplin useable at least. Hopefully this is something that will be fixed in a future version (might need to wait for an Electron update).