I found that the amount of CPU time gnome-clocks uses for rendering a
running stop watch or timer and even more what it causes Xorg and in a
smaller way gnome-shell to consume is simply way out of proportion.

The combined (gnome-clocks + Xorg + gnome-shell) CPU usage on this
laptop as reported by top goes over 50% (of one core, that is), same on
my desktop machine (intel and radeon graphics, respectively). This makes
it somewhat unsuitable to run on battery power.

Given the relatively simple graphic effects it displays, I would expect
much less resource consumption.

forwarded 775242 https://bugzilla.gnome.org/show_bug.cgi?id=738469
thanks

This bug has been reported upstream by Michael Biebl a few months before
I submitted my bug report. So let's just use his report as the forwarded
address.