It probably is not this, but maybe you can try it. For a while HP servers had the extremely annoying property of the boot being rate-limited by the serial console speed if you had been unfortunate enough to enable it by default. You would not notice anything other than the slowness on a video monitor, but if you plugged a serial cable in you could see it struggle.
I've never seen this on anything other than HPs and for good reason. mcl
