I'm using Firefox on Fedora 31, but this seems to be an issue across multiple OS versions. The machine is a Dell Ultrabook with a Core i7 processor and 16 GB of RAM.
It's odd. What desktop environment are you using? Mine's from 2014 or 2015 with xfce. spell check is enabled from the looks of it, but only comes in when I press "space".
5 frames / 160ms for the key to appear at the start of a chat line.
3 frames / 100ms for the next key to appear
Firefox in a plain text box takes 2 frames / 65ms
What OS you using? I haven't noticed an issue on the rare occasions I use my (7 year old) mac