Kurtiskain, that's something I've been looking into myself, but have never been able to find an answer to.
it seems obvious that the BIOS would put no load on the CPU, but on some boards I've noticed it seems to be stressing it.
Examples: my main system, seems to be a small load as my CPU gets to the mid-30s in the BIOS, under full load in Windows about 46, and idle in Windows high 20s. My laptop idles in Windows in the 30s, load at 60-64, in the BIOS there's no way to check the temperature but it does feel much hotter than idle in Windows.
The original reason I looked into this was my Athlon XP machine, it was in the BIOS at over 50C which didn't seem right to me, with a cold room and open case. It was lacking a HD at the time and now is missing a cooler, so it doesn't have an OS on to compare against. That's what makes me think that the BIOS does put a load on the CPU.