Why does gnome-terminal takes so many resources to perform trivial tasks such as
scrolling text up?

Experiment: Run gnome-terminal and xterm side by side.

Do a "cat filename" on a very large file in each one of them and see which one
is faster.
xterm can be faster by orders of magnitude (i'm not making things up).

Compile an application that makes gcc very verbose in each terminal. With
gnome-terminal, the overall system load is significantly higher.

Run a "tail -f" on a log file, while at the same time running a time- and
load-sensitive application such as any digital sound recorder (Ardour with JACK)
or sequencer (Rosegarden, Muse).
If "tail -f" runs in gnome-terminal, and the time-critical app does not run with
real-time privileges, parts of the sound stream will get dropped. The drops
occur at a much lower frequency (if at all) when running "tail -f" in xterm.

I mean, gnome-terminal is nice and all, but why is it such a huge resource hog?

I am not the first one to raise a flag on this issue. gnome-terminal essentially
makes it difficult to do time-critical work on a Gnome workstation.
I actually avoid using it.