Here is a benchmark between most terminals available. The benchmark is quite simple: cat on big RFC (SIP).
The winner for speed is gnome-terminal, with konsole quite close (only 2 times slower :) ). Regarding memory usage, if you have only one terminal konsole will use less memory than gnome-terminal, but starting at 3 gnome-terminal will use less.
So, thanks a lot to GNOME hackers for improving gnome-terminal which was quite slow few years ago and is really fast nowadays !>