Please keyword app-misc/tmux-mem-cpu-load-2.1.0. It works here on ~amd64 and amd64 systems flawlessly (and can hardly break anything, really)
Arches, please stabilize =app-misc/tmux-mem-cpu-load-2.1.0 to test, just run tmux-mem-cpu-load :)
amd64: ok
amd64 ok
amd64 done. Thanks Agostino and Ian
x86 stable, all arches done.