Forgot to say: to reset SMC on iMac just pull out the power cable and wait a little bit.
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
Same problem with iMac 21,5 2017, started right after the Big Sur update, can't believe they let this happen in official release 😞