Architect preview causing crippling lag

Ever since I first tried the Architect editor, making arcs has been a pain because if I turn on the 'start animation' button on the preview for a NPC, despite said NPC being tiny, it causes crippling lag that slows the game to slide-show speed. It used to stop when I got the animation to stop, but today it kept going and the only way to stop it was to reset. Needless to say, this makes the MA very difficult to use.

Is this just me, or a known bug? And how can I fix it, if I can?