how much RAM do you have? all of those programs suck up increasing amounts of memory over time, if it gets to the point where your RAM is exhausted and the system has to start paging out files to the hard disk it will start to slow up.
Generally speaking, if when you have a look in Utilities > Activity Monitor, down the bottom, at page in/outs, and the number of page outs is any reasonable percentage of the page ins (like more than 20% or so, although I'm just making that figure up actually - bound to be someone here with a more precise definition) you probably need more RAM