This box currently has 62 active processes and is running fine. (I even know what some of them are!) Statements like:
The fewer processes running, the better.
are simply nonsense.
If something eats vast quantities of memory and actually uses it, so as to cause paging, then that can result in dramatic slowness, characterised by continuous disk noises and the disk light permanently flashing. Culprits can be anything, but common ones I've seen include antivirus software and
anything written in Java, particularly if it has a Swing GUI and includes memory leaks (don't let anyone tell you that you don't get memory leaks in Java "because it's garbage collected" - this too is simply nonsense).
On the other hand if something has vast quantities of memory allocated
but isn't actually using it then that isn't slowing the machine down.