Hi Nick, it's the apparent intersection snap kicking in - when a helix like that is projected down to a plane for generating the apparent intersection snap it produces a flattened curve that loops around over and over on top of itself and that tends to cause the intersection mechanism to do a lot of extra work.

Here's a couple of previous posts about it:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3252.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=3129.1

The problem should go away if you disable apparent intersection snaps.

- Michael