Hi, I too am one of what seems to be many people who migrated from Evernote to Notes when iOS 11 was introduced (I have 2962 notes, many of which are images converted from stylus-written penultimate notes). I too am experiencing frequent systematic temporary freezing (anywhere from 2-45 seconds), app crashes, and glitches with stylus input (either pen marks, or eraser marks not visually being reflected, but upon restart of the app, actually appear).
Two months back I was on a chat with Apple support on this issue. He recommended numerous tips that temporarily reduced the frequency and severity of the problems, but importantly did not fully and permanently resolve the issue. But these tips are still useful for me when I'm walking into a meeting and need to ensure the best chance that my Notes are usable for note-taking.
The steps Apple Support told me to take are as follows:
1. Turn off background app refresh for Notes (under settings->General).
2. Turn off background app refresh completely
3. Force close all background apps (theoretically the way apple implements iOS multi-tasking, this shouldn't matter, but the Apple Support told me to do it, and it actually seems to have an impact).
4. Finally with all the above steps taken do a hard reset
This significantly helped to the extent that I thought the problem had been completely eliminated. But alas the freezes resumed, though with less frequency and severity. But the trade-off of not having background refresh for Notes was proving problematic for versioning reasons (I use notes on my phone and on my macbook), and having background app refresh completely off was a non-starter.
In practice, I now routinely close all background apps (something that used to annoy me when I see others doing it), especially when I am using Notes. Ahead of a meeting where I plan to take notes in Notes, I temporarily shut off app refresh (and I bring a physical pen and notepad as backup).
This still falls short of being acceptable for a consumer that has literally spent tens of thousands of dollars on apple products over the years, but it's a stopgap, _assuming_ that Apple intends on resolving these bugs (which, given how they are pitching the iPad Pro as a professional working platform, I'm shocked that they haven't been more proactive at it).