2 things I suggest
1. reload the OS first, this can solve many issues, but do a archive and install, even still, it WILL not hurt to clean it, I seen what dust buildup does to computers(laptops especially!), it's not pretty
2. This is a older computer, I suggest taking it apart and cleaning the heatsinks, then use a good quality thermal paste, it could be overheating, thus in the "throttle" mode for the processor to protect it
Here is the guide
MacBook Pro 13" Unibody Mid 2009 Heat Sink Replacement
The throttle mode is a protective measure for your processor in case it gets too hot, and will "throttle" at a certain temperature so the processor does not burn up and fail, so it will benefit from a cleanup and good thermal paste if the OS is no help
Try this first, then tell us what happens
If i get that Dell inspiron 1525 from my friend, trust me, it's a GREAT example of how simple it can be to fix overheating issues, and the best example i'll ever have