I've come up with a solution to disable auto sync on the desktop app, using a plugin which I've vibe coded, to work around the issue.
You can download the plugin here from the releases section:
See the README for additional details, and a suggested workaround on mobile. I've also attempted to publish the plugin to the plugin store as well, though I'm not sure how long that takes to appear.
EDIT: It's now available on the plugin store as well
I have to say I'm surprised that this seems to work. And the general quality of life performance improvement from the sync-as-you-type being disabled is absolutely loud and clear.
To me though this just proves the point that Joplin really should probably have a dedicated setting to be able to disable the sync-as-you-type until its performance can be properly improved (that is, if it ever can).
Don't get me wrong, I'm quite glad a workaround hack is possible, this is quite an interesting workaround and I want to thank you for putting it together. But for me I seems having a dedicated setting would be the simpler & cleaner solution.