This question may be better asked on the Raspberry Pi forum but I'll try here first.
I generally update my RPi4/2gb & RPi5/4gb every time a new le12 nightly comes out and a while ago observed there was a noticeable difference in the time taken for them to update.
Finally got round to using a stopwatch to check (these are times from when the "Decompressing image file..." message first appears):
RPi4: time to decompress = 27 secs; total time up to the "System reboots now..." message = 40 secs
RPi5: time to decompress = 54 secs; total time up to the "System reboots now..." message = 78 secs
So double the time for a much more powerful cpu - unless I'm missing something glaringly obvious. Double the ram shouldn’t make any difference should it?
I'm not really bothered about waiting for it to update, more curious as to why a RPI5 is so much slower.
p.s. le running on a usb 3.0 stick in both cases (similar read/write times).