Seems VK is right... Darken.shp really is the main cause for the slowdown, although there is still a slight slowdown even when darken.shp has been made completely transparant (filled up with color#0)...
Apparnetly the gclock2.shp anim has a fixed number of frames and removing some frames just make it play shorter, but whatever is being produced will still be building a little while after the gclock2.shp anim has stopped playing (so that obviously isn't much of a fix after all).
I just tried modifying the darken.shp anim however and found that the fewer visible pixels it has, the less/shorter the slowdown will be.
For me the slowdown is only noticable when playing on gamespeed 6 offline however, so I personally don't require a fix for this. I attached an Eache02.mix file with a darken.shp file inside for whoever does experience the problem more severely (darken.shp now just looks like a small X in the middle and a a border against the edge of the cameos). If this still doesn't get the game to run normally when building stuff on gamespeed 5 (although it should at least improve it), you can try making the anim entirely black instead.