I have the exact same computer you have, and nothing's sloooooow here.
So: new CLEAN install, this time booting from DVD and erasing everything again. Never installed Logic nor any update.
That's your problem right there. 10.5.0, .1, .2, had a lot of problems, graphic issues, etc. but if you had done all of the software updates to 10.5.3 or 10.5.4, including all the graphics updates and what not, you probably would have seen an increase in performance back up to what you were used to in Tiger. I upgraded from Tiger on the same machine, and with 10.5.3 and 10.5.4, it's fast and crisp, and runs Logic really well.
It is good to wait a bit with a new OS to where they work out a lot of the kinks, but once you decided to make the plunge, you have to get yourself up to the latest version, or else all the problems will still exist.
When I was running Tiger, I thought 8.0.2 was WAY better than 8.0.1.
Just my .02 🙂