I updated Premiere Pro CS4 (and the whole CS4 Suite) to 4.1 and had serious performance problems. It crashed when I saved a project, also after a rendering a preview, the red bar (for rendering on the timeline if you put an effect on a clip) disappeared and was suddenly back again and everything got very very slow and crashed completely after minutes. I tried it without any external harddisks, like my raid1 over SATA, put a clip on C and also stored the project on C, but always the same. After 2 days I went back to my 1st setup, reinstalled 4.0 and did a update to 4.01 - everything works fine again.
Hardware: HP8600 workstation, 4GIG RAM, Nvidea grafic card, Windows XP, Motu V4HD and Blackmagic (for AFX), all latest drivers, no other software on this system.
The problem occurred not only with MOTU projects, but also with all other I tried
ps when I did the update to 4.1. I got a error message: Adobe Output Module 2.1 could not be installed
sorry for answering so late. I've got the same problem and I'm using a MOTU v4hd, too. I contacted motu and they confirmed, that the bug is in the driver, which does not work in cs 4.1 anymore (same problem with Matrox, so no motu fault). They are currently working on a driver update and recommend, to use cs 4.0.1 until driver release is ready.