Easy. It is the hard drive. Hence it is a 5400RPM drive, it's a simple laptop drive which won't perform very well in any situation. They are made to save power.
Get a 3.5" closure, and a WD Caviar Black hard drive.
OR if you can't buy a new closure, get a 2.5" WD Black Scorpion.
Both hard drives are cheap, and designed to be perform very well in a desktop environment / demanding situation. Personally I use both. Caviar in my desktop, and Scorpion in my laptop. Both are blazing fast. (On my laptop, I didn't notice any overheating/too much heat after installing the HDD.)
NOTE: I'm not a WD fanatist, or whatsoever. I just couldn't find any other hard drives designed to be like this. Only found simple, normal ones. If you know about any "high-performance" HDDs at other manufacturers, just link them / modify my answer please.
About the USB2 interface: If you buy a new closure, you will have a 3.0 closure anyway. However, if you keep the one you have now, don't bother about the interface. It only matters if you keep copying straight. Like a huge VM file. If you just work on it, run your VMs, it won't matter.