Check to see if there's enough free space on the startup disk.
Right or control click the MacintoshHD icon. Click Get Info. In the Get Info window you will see Capacity and Available. Make sure you have a minimum of 15% free disk space. Freeing Up Hard Disk Space - Mac Guides
Some have found that running a disk maintenance program periodically can help with performance. I use OnyX and can recommend it. It's available from: http://www.titanium.free.fr/download.php . Run the tasks on the cleaning and maintenance tabs. With any disk maintenance software, you should have a backup before using it.
As Carolyn mentioned, free disk space is important. You need 5GB minimum, with 10% to 15%+ better.
Check if you have enough memory for what you run. Bring up the Activity Monitor (on your hard drive in applications/utilities), click the system memory tab, and look at page ins and page outs. If page outs is 10% to 15%, or more, of page ins, you can use more memory for what you run. A page out is when OS X writes memory to your hard drive, to make room to run something else. Too much of this can slow down performance.
Have you downloaded any new software recently? If it's not 10.5 compatible, and loaded by the system, that could slow things down.
Create a new login ID with no login items and see if that improves performance. If so, you have one or more login items that are causing performance issues.
Also, see this FAQ: http://www.thexlab.com/faqs/performance.html