On Android with WebDav sync I also have the problem that the app does not react for about 7 seconds at startup through the sync and on the intervall sync. So I increased the sync interval, so that the sync is not synchronized so often.
I'm in the middle of re-syncing everything from scratch, trying different settings as an experiment. Did you import from Evernote as well? Markdown or HTML notes?
I have imported 5235 notes with a size of round about 4 GB from evernote as markdown.
It seems that File System sync with 10K notes and 12GB my UI freeze was 15s, similar to WebDav. I never expected File System sync to introduce performance issues.
Now I have 10K notes and about 8GB of data, and with Dropbox sync I'm seeing no UI lag at all. It's absolutely fantastic by comparison. File System sync has some sort of bug, in my opinion.