I think the best way to put it is:

If you have a well to over-powered system anyway - you'll never notice the differance...

If you have an average built or couple year old sytem - you'll notice a little bit of a differance, but not much...

If you have a system that is outdated or so low on resources its already slow - you'll likely notice a big differance

(And then obviously many varying degrees of severities in between each)

As they tend to effect CPU and Memory more then actual download/upload speeds, but if your CPU is already being overworked, or your systems already low on resources - well then the added load is going to be quite noticable... So obviously, the more overpowered your CPU and the lighter your resource load, the less your going to notice it...

and not to knock what ericc said - but

This is in regards to software firewalls which other then some of the most basic ones, do something differant then hardware firewalls do - which is why its always a good suggestion to run BOTH...

RRP