As for the problem itself: By unchecking non-essential processes that run in the background we speed up the computer. less processes equals higher speed. But not all processes are equal. some use considerably more resources then others. You should note that every program that runs in the background can be activated through a desktop icon or through "All Program". The only programs that must be started immediately are antimalware programs such as AVG and any firewall other then Microsoft's.
In addition, there is always the possibility of a corrupted program. When you open it in Startup you have it running all the time, slowing the computer or causing other problems. The way to pinpoint such a program (if there is any) is re-check startup items one at a time and see which one has an unacceptable influence on computer performance.
As a rule you should keep running processes to minimum.