I am also experiencing slow boot times, on a 2017 4k 21inch iMac, running MacOS 12 Beta 2. It can take up to 30 mins to get past the Apple logo, with the progress bar stuck at around 50%. Strange as Big Sur booted in less than 30 seconds so not sure what is causing the issue. The same as you, the diagnostics came back fine… How long did your Mac take to boot in the previous beta?
JhodgesDigital
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