One other thing, can you think of why my pc is a bit slow responding? For example, moving the mouse pointer around the screen, opening folders/files etc.
That can definitely be one of the noticeable effects of a background process which is taking CPU and/or memory resources away from your applications and user-initiated events such moving the mouse, opening folders, etc. More specifically, a symptom like that usually manifests itself in "spurts". That is, your mouse may periodically lag for a moment, but then return to its normal behaviour; if the lag is constant, that may be indicative of some other issue.
I understand that by checking the items then that would speed up the time it takes for the computer to boot up, but would it also speed up the computer generally??
Technically yes- because most of the startup items remain active in the background (hidden) while Windows is running. Whether or not the actual performance increase gained by disabling startup items is actually noticeable to the user is another story.