When a large (i.e. 1000+) amount of lines are collapsed in the script editor, performance tanks. If the fold is expanded, performance is fine. @Myaxp let me know he was having this issue recently, but I vaguely remember this being around for a while. This happens 100% of the time and across multiple machines, but has only been tested on Windows 10 images so far. The behavior can be reproduced using the following model:
FlowerDance_Hard.rbxmx (51.0 KB)
Insert it into a blank place and open it. Try to drag select text and notice that there are no performance issues. Collapse the HitObjects table on line 28, try drag selecting again, and notice a severe drop in performance.