Hey,
Have you installed any new programs recently? Open up Task Manager and make sure there are no applications hogging system resources (Check your CPU usage). Then download Realtemp or Core Temp and check your temperatures. Please post them here, if your temperatures are too high it could be that your computer is overheating and is throttling causing reduced performance. Have you moved any components recently? Check that all the fans in your case are running properly and that the CPU fan is not damaged or dead.
Hope this helps!