MacBook Pro i9 (16-inch, 2019), 11.2.3 Big Sur. Definitely seems like it should have enough grunt to handle a 10MB text file, but not any more. Currently been sitting for a few mins at 100% CPU, lagging out my keyboard input in my browser, apparently due to the beachball twirling in my TextEdit window. I end up force-quitting it about once an hour. The most frustrating part is that this was never an issue - just last year I would've opened gigabyte-sized genome fasta files or various other 100MB text files and scrolled through it with no issue. Was always impressed with how much better than Windows notepad it was. Note that I've turned off all spell checking in preferences etc., so I don't think it's related to that.