Seems like it only applies to some source editors, but you're right, it shouldn't happen like that. I only have Microsoft Edge on my PC, which is my "default" browser.
I already reported this bug not too long ago, and prior to that, I have also been researching this in Community Central to see if anyone else was having the same thing. I first thought this was an issue with my browser, but until then, I discovered that it's a bug related to Fandom. I even tested out opening and editing a large Wikipedia article (without saving), and I did not have any issue with that at all. I even also discovered that any medium-sized or "slightly" big articles can also cause your browser to lag (if that is happening to you also).
As I mentioned before, Fandom is already aware of the issue and are in the process of fixing this bug. I bet it has been reported several times already.