I definitely can feel the lag in normal usage, and it's stressing me. Having tried various terms, I've always quickly returned to xterm with bitmap fonts - even before I'd heard of latency measurements being done on terminal emulators. xterm feels almost instantaneous. Gnome Terminal, for example, always felt sluggish. Not having done any measurements I couldn't tell what it was (and didn't think that other terminals could possibly have noticeably slower latency) - and I still think there can be other contributing factors, like usage of Vector fonts and aliasing. Or, as someone else points out, it might be more jitter than lag.

I also never liked GNOME 3, and suspected it must have something to do with it being a compositing WM and the latency it probably adds on top.