I'm just curious, if you're using a fraction of the line constantly (say 50%), would that slow down the response times? what about if you're using 50% of the CPU (but it's not hitting 100%)?
It's possible, yes (for example, your disk or network I/O could be saturated without the CPU hitting 100%)... but has to many variables to give an absolute answer. In reality, it's probably not going to be noticeable, but it's never a good idea to run a server that runs close to 100%. For me personally, I generally like to keep my servers under 10% load, that way it can respond easily to peaks times.