It's not really UEFI issue, but what modules were loaded, how it was configured/wired up - and sometimes it ends up serializing on slow boot of some components (like DDR5 memory), and weird crappy add-ons from manufacturer.
replyOn many machines with DDR5 you get the lengthy memory training every boot and that comes on top of the already long UEFI initialization time.
replySurely that could run in parallel with initializing everything else, with the amount of CPU cache modern CPUs have?
replyAlso the soldered ram couldn't have changed from last boot, so reuse the last training? That's exactly what I mean.
The memory training happens before UEFI is initialized proper, though it provides a minimal framework for handling such parts - it's mainly so that every bit of very early code can properly pass information later on.
replyAs for reusing the training, that's what normally happens, but there are various things that can trigger retraining beyond swapping anything in memory modules, and it's perfectly possible the vendor somehow managed to write an add-on that triggers that on every boot (just like intel managed to wreck NVRAM on every netboot, for example)
The training reuse is already happening on machines with regular user-replaceable DIMM modules. It's why there is such a scary long wait on the first boot of a newly-assembled system, but not subsequent ones.
replySo even if it can change, starting out with a simple "Are the same modules still installed?" check saves a lot of time.