A bottleneck will always exist due to some component, because we will always have bandwidth limits. Usually it's the CPU or motherboard.
I forgot the specifics, but there's a difference between lag caused by a CPU bottleneck and lag caused by a GPU bottleneck which is visible. I think it has to do of whether the entire system runs slow, or if just the graphics processing, but I'm probably wrong. I haven't had to look at bottlenecks since Assassin's Creed III.
Should be noted, most games are CPU-intensive for mind-boggling and stupid reasons.
