The fact is that Mac OS X is good at of taking care of itself, if there are performance issues, there are a couple of things you can check:
- Check your Login Items under System Preferences > Accounts to see what you have automatically launching.Also check /Library/LaunchAgents/ and /Users/yourusername/Library/LaunchAgents/ for items launching at startup
- Launch Activity Monitor and change "My Processes" at the top to "All Processes". Then see what may be consuming your OS X system resources.
- Check the System Memory tab at the bottom of Activity Monitor to see if you have excessive "page outs", a sign that you may benefit from more RAM.
- Check if you are running any poor Mac anti-virus programs as they may take large resources.
If you don't want to do these manually or the above checks still won't help, let the MacBook speed up tool http://www.speedupmacbook.com/ perform a health check of your Mac and fix relevant issues.