Comment by bjonnh

Memory initialization is really slow. On servers there is that, plus the fact that you usually don't want to do a fast ram check like you can afford on a laptop. Add to that initialization of the remote management, SAS/raid,... A server does a pretty thorough quality control when it boots. Also if you have a lot of mechanical drives, there may not be enough power budget to spin them all up at the same time, so they start progressively.