Leopard is quite snappy for me... much more responsive than Tiger.
I suspect you are having some issues. Try these things:
1. Open Activity Monitor and see what resources are being used.
2. Check your login items under System Preferences > Accounts > Login Items for any third-party applications that may be slowing down your system.
3. Repair your permissions.
4. Clear your caches.
These are basic maintenance suggestions, but if the problem persists, I would suggest installing Leopard from scratch as a new installation and then slowly moving your data over until you find the culprit.
I base my belief on the fact that, as Siracusa noted in his review, Apple engineers worked hard to make Leopard more responsive than previous generations of OS X. It makes little sense to me that your system has become slower, as Leopard, on average, does not use more of your system resources than Tiger.