Problems related to device drivers, specific programs etc might cause systems running Windows 7 to be very slow in general, can cause start up or shut down issues like taking too long to boot up or turn off and can also degrade overall performance.
Here is a very basic way to check whether any performance issues are detected in Windows 7 :
1. Go to Control Panel > Performance Information And Tools.
2. Select Advanced Tools.
3. Check out if any performance related issues are listed in the Performance issues column. Click on the listed issue to find out more details (in this example, the display resolution set is causing a little sluggishness when used for extended period of time).
This gives a general idea of what can be hampering the system performance so that one can start troubleshooting using these listed issues as a starting point.