No, it's just 'Sprite overlaps Sprite' requires N^2 checks. The first event picks 10 instances then needs 10x9 = 90 collision checks (not 10x10 since it doesn't have to check if it overlaps itself, only the others). The second does the collision check first needing 20x19 = 380 collision checks, then filters them down to the 10 with the variable set. I presume your layout is the size of the window? In this case collision cells have no effect. They work best when objects are distributed over a large layout.