And most machines don't even touch that limits. I remember some presentation about it [1] and actual limits aren't even touched, especially on servers.
From what I remember "hard" limits are CPU/DRAM-memory initialization and speed of read from flash chip storing, and sources of lag include firmware from add-on cards just being slow (if RAID controller takes 30 seconds to return, and firmware is not running initialization in parallel, that's your extra boot time. Or stuff out of left field like "IPMI controller logs stuff via serial so if you print too much text it slows down". Most BIOSes do things painfully parallel too.
* [1] https://www.youtube.com/watch?v=fTLsS_QZ8us
