I get just over a second for the 1st transition each session then well below 1 sec for subsequent PiP swaps.

I have an Athlon X2 4600+ 2.8gb Ram (bloody mobo) and a Radeon X1950XT it ain't exactly cutting edge
off this I run the following sessions
1x 1520x1200
4x 380x300
on a 24" LCD