Thank you for your reply Ashley.
I've followed the advice on the performance tips page and I'm running a system with a i5 2500k clocked at 4ghz, 8gb RAM and a GTX560 card with the latest drivers.
Though the performance issues were quite logical as I had several hundreds of object instances and a lot of collision checking going on.
But I've redone almost everything now when I'm familiar with the software so that it works more efficiently and uses about a third of the object amount. :)