I’ve noticed slow down when lots of instances of object that is just graphical object (such as animated candle to make level not fill empty), even with no event code referencing it or using it. It was when I was developing Candy Man Saga for that jam that I noticed this (wall backdrop objects did it), it’s especially visible on level 8 I think.
I think Gamedevelop’s rendering pipeline may be causing of this since objects didn’t have any code associated with it (no event code, no automatism) and after ripping off large chunk of wall, game started to run faster.