(This might well be a duplicate of one of the other composer related performance issues, but since I noticed a very specific trigger condition and didn't find another one which would have matched the symptoms exactly, I decided to file a new bug.)

When I have an external (4K) display connected to my laptop, composing messages with Evolution is practically impossible. It takes around five seconds for the message editor are to load, and once it does, it takes around three seconds from the moment I hit a key on the keyboard until the character shows up in the editor.

It doesn't seem to make any difference whether the editor is in plain text (as normally) or HTML mode.

Once the external display is disconnected, the composer works as expected.

If there is anything I can do to help tracking down this problem, I'm happy to do so.