Is this a workaround for the flipbook having no sprite based collisions? So are you having a sprite component for each frame of animation in the same actor and then switching though them to be visible frame by frame? OR are you using each sprite as a separate part of the same object (i think from your post that its the latter). It might be more performance friendly if you made your collision out of capsules and boxes. I know it’s only simple custom geo but its still geo collision which is known to be more expensive.
Also have you constrained the physics engine to be 2d? it might cut physX testing in 1 dimension and help things.
I haven’t played too much with the 2d collisions so feel free to ignore me though!