I initially thought this affected MSE only, but now I started to face the same issue on other SE sites.
I've encountered/reproduced only on Stack Exchange on Firefox for Win 10.
Place cursor at the start of a paragraph:
Press Enter twice, and try to move cursor up using the ↑ key:
Huge delay is faced when moving the cursor up one or two lines.
I asked in chat a couple of days ago on how best to report it, so if there's any data the team here needs, let me know.
Note: I can repro most of the time, and it only affects (as far as I've encountered) jumping to blank lines.
Update: Based on two comments, the issue affects Firefox in general, though as demonstrated in another comment, it doesn't affect Wikipedia; for the time being I've rolled back the addition of the bug tag until the issue becomes more clear.