Same problem with iMac 21,5 2017, started right after the Big Sur update, can't believe they let this happen in official release 😞
AndreyKitaev
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Resetting SMC and NVRAM helped! iMac started to boot as usual (about 20 sec) and all programs appear ultra fast. Hope this helps you
Forgot to say: to reset SMC on iMac just pull out the power cable and wait a little bit.