You could also trying running the maintenance scripts. There a 3 scripts which are designed to run automatically early in the morning with the computer still on obviously. Daily, Weekly and Monthly. They clear up temp files or something and sometimes help with a slow down like you described.
I recommend this utility,
Onyx. Take time to read the readme file and be aware of what this little app is capable of. She's not to be messed with.
Includes some nice misc. parameters like locking dock icons, placing two scroll arrows at both ends of the scroll column etc.
"It allows you to run misc tasks of system maintenance, to configure certain hidden parameters of the Finder, Dock, Safari, Dashboard, Expose, Disk Utility... to delete cache, to remove a certain number of files and folders that may become cumbersome, to see the detailed info of your configuration, to preview the different logs and CrashReporter reports, and more."