Rube,
I have used PI very happily for several years. When I switched to WM5 all of a sudden it became slooow. My biggest complaint was the startup speed (abt 10-13 sec) but browsing between months sometimes got sluggish, too.
As I found out, there are two main factors.
First, WM5 is by itself slower than WM2003, and WebIS basically said they can't do anything about it. Whether or not it's all OS fault or the software code is also to blame is an open question, for me.
Second, the database size is very important. I used to sync all my appointments (about 3000 spanning 5 years), when I switched to syncing only 3 months it became much faster.
In your case, I'd suggest syncing only a few months of past appointments. Also, I'd use Month View, not Week View, for browsing days. It's both faster and easier to read information from. You can zoom in on a week in Month View by tapping your stylus on a day and dragging it to other days you want to see.
Finally, take a look at Pocket Breeze at
www.sbsh.net, it may or may not work out better for you (that's what I ended up using). Just make sure to use trial this time ;)