Yes, the bigger the runtime, the slower Poser runs. 

This tutorial tells you how to set up multiple runtimes, offers advice on organization, and explains what the advantages are:

http://forum.daz3d.com/viewtopic.php?t=9060

It was written for Poser 5, and but  applies to Poser 6 as well.  (And probably Poser 7, too.)