The slowdown is due to construct recalculating the collision mask on resized objects for each frame. It's from a fix that I made in construct since 0.9984. Prior to that, the collision mask wouldn't get updated unless the object was resized or rotated. The only drawback is it seems to be one of the main sources of slowdown encountered in projects. The slowdown is avoided if collisions are not continuously compared with animated, resized objects.
Anyway in your case the solution is to have a separate object that handles all the collisions.
Here is a fixed cap
http://dl.dropbox.com/u/5426011/examples2/fixedreadthesigns.cap
saved with 0.99.94
Edit :
DravenX's solution of setting the guy's collision to box will work, but by looking at your Hitbox animations you want the hit box to be smaller than the sprite. In which case another object should be used.
-cheers