I confirm the above issue posted by KillerBob. I updated my iMac 2019 this morning to Big Sur 11.3 official release (20E232). My boot time is now close to 4 minutes, while it used to be 20 to 30 seconds before with Big Sur 11.2. The boot process stalls right in the middle of the progression bar. The Thunderbolt bridge is still present in my Network settings, but it is listed as "not connected" with the red marker, while it's been working flawlessly for the last 3 months with my QNAP NAS, which doesn't states "Thunderbolt device is connected" anymore while my iMac is booting.
gr-_-
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