Install istat menus so that you can keep an eye on ram, hdd, and cpu utilization.
Link:
http://www.islayer.com/
There are a myriad of reasons why you could be noticing a slow down. Which version of OSX (10.5.2?) are you running? What is you ram utilization during normal use? What is you cpu doing under normal use? How full is your hdd? The program I just mentioned should help you determine some basic info as a starting point and it is highly recommended.