Very weird. It's not the first time I'm reading this. Not so over here (dual core iMac from 2011 - Not a state of the art computer!)
I can open about 20 and my CPU goes up to 20% or so (and we're talking Ableton Live, which is very CPU unfriendly).
I think Jeroen is not active on GS and guys like you should help him out by reporting him these issues with your specs.
He's very, very responsive and fast at updating his plugins. He can't fix possible issues, if users don't report them