I'm afraid I have to agree on the performance issue. I've reverted back to CS3 because of it :-(
I can have the previous version open for days, while working on big files and no problems. CS4 on the other hand hangs after a few hours.
I believe Adobe may be working on a fix. At least I hope they are. I liked the updates to CS4 but right now, it's totally unuseable for me.
I'm not even sure that these performance-issues are directly related to specs. I run FW CS4 on both a desktop and on a laptop, and contrary to expectations, it performs much better on the laptop.
There could be other apps, certain fonts or system configuration that decrease FW performance. But I agree that there is much space for improvement there.
I run both versions of FWs on my laptop so the specs are the same for both versions. My problems as as described above, so it has to be CS4 itself :-)
The only thing I've left to do is update my graphics card driver as I also have an issue with opening any files in Photoshop - totally unuseable.
However, we'll have to see whether this is the problem as the laptop is fairly new, but you never know :-)