This is an additional report of this "Invalid" issue. https://bitbucket.org/site/master/issue/5511/commenting-on-large-pull-request-is-very
As above, large pull requests create crippling performance problems in the browser. One of our projects has several pull requests which are so large that even attempting to scroll the page results in a lag of 5+ seconds at times. Typing comments is as bad if not worse.
I cannot link to any of these pull requests as our repo is private, but it appears to be an endemic issue which I deal with on almost every pull request I review. We're using relatively powerful machines (a few years old)