I've never heard of or experienced lag. I have 2 kvm switches on both my bench and my server desk (both 4 ports). One's a zonenet and the other is a linksys. I would recommend them to anyone.
One of my friends has one that has a floor stomp pedal which is really cool. I can't think of the name but yea.
Back to the original query, kvm switches are very simple by design and do no processing of signals. If the one you're looking at does, then it's not worth it. Just out of curiosity, is this for gaming?