Hi greyowl,
Well more or less everybody (or they must be very advanced users) need to have some sort of firewall installed (hardware or software fw). Some firewalls are lean on the cycles, other can make your comp a bit of a slow starter, but what they defend against (malware and especially forms of spyware and adware) is what is really slowing your computer down or bring it to a grinding halt even, or your computer will rank in a zombie pack and is not owned by you anymore, what do you prefer?
polonus