What does the performance of a hard drive affect?

The three most notable things that hard drive performance will effect is:

1. The start up time of your computer. As the information need to run the OS must be transferred from the hard drive to the systems RAM.
2. The start up time of programs.
   
3. If you are using almost all of your RAM space the system will use your hard drive to store information normally keep in RAM. In this case your hard drive performance will effect your entire system, but more ram will correct this problem better then a faster hard drive.