Performance issue with 3.21.x

I'm using meld as part of my git workflow, and there seems to be some performance issue in 3.21.x when dealing with large files. A nice example/reproducer is config.sgml in postgres repository:

$ git clone https://github.com/postgres/postgres.git pg-test

$ cd pg-test

$ git difftool 9ccdd7f66e 9ccdd7f66e^

If you try to type anything, it'll take very long time (a second or two) to actually render, which makes this rather unusable for reviewing patches etc :-(

If I switch to 3.20.3, this works perfectly fine - it renders immediately. I can reproduce it on 3.21.0, so the problem seems to be somewhere in between.