@dario2994 Yes, I confirm that you are having the same issue as me.
can you build Scite (the Scintilla reference editor) version 3.7.5 (to match the Scintilla used in Geany, not the latest) and see if the problem occurs there as well?
@elextr Yes, I installed Scite 3.7.5 and it has exactly the same issue as geany. Which makes sense. Slow update when going to a new line, lag when typing, etc.