I recently installed 5.4-stable on a machine with an intel graphics
device so I can tinker with the framebuffer console.
[dmesg]: http://pastebin.com/raw.php?i=eKgZzNa8
I notice that when running tmux on the console, output to the screen is
very sluggish and text seems to scroll with a wave-like effect.
I built the kernel from source a few times to collect some rough data
that might demonstrate the extent of the performance problem.
From /usr/src/sys/arch/i386/compile/GENERIC.MP
# framebuffer console
time make -j 4
4m43.97s real 8m20.15s user 0m53.01s system
# framebuffer console with tmux
time make -j 4
22m53.03s real 8m22.85s user 11m3.58s system
# ssh from remote
time make -j 4
4m37.65s real 8m16.88s user 0m45.16s system
# ssh from remote with tmux on local
time make -j 4
4m40.15s real 8m16.46s user 0m45.28s system
