Late 2009 Macbook lagging, becoming hard to use

I am running a late 2009, 13-inch Macbook with a 2.26 GHz Intel Core 2 Duo Processor. I am running Mavericks OS X 10.9.4 with 4GB of Memory. I have a 250 GB hard drive, with 36.88 GB free. Over the past few months, I've noticed that my computer lags really badly with just about everything. Opening up applications takes forever, and then once they are open, they freeze up constantly. For example, opening safari windows often leads to freezing (spinning beach ball) and slow loading pages. But it's not just Safari...it happens with mail, iTunes, etc. Startup also seems to take forever now. I've run the basic Onyx Maintenance tasks every so often, but it doesn't seem to make much of a difference in performance. Any tips on what I can do to speed things up and reduce the lag and constant freezing? I've seen various threads mentioning things like doing a clean re-install of Mavericks, but then I've seen other threads that said there should be no reason to do that and that I shouldn't need to do any maintenance! So I'm at a bit of a loss as to what is the best course of action. t can provide any additional information that would help.

Thanks!

I wouldn't consider myself a heavy user and, overall, it serves me well. So if I would probably continue using the machine for as long as I can, as long as it is working ok. Right now the lagging is the only reason I would consider replacing it right now.