The exact problem happened to me and is still an issue. I have a 2017 iMac 27 inch. 40GB ram and a Fusion 27 hard drive. I also have a 15 inch MacBook pro. I updated both of them on the same day to Big Sur. My laptop has no issues but my iMac takes almost 10 minutes to startup. It is May 6th and I have not found a fix yet. I re-installed Big sur but the same issue. I am going to try and go back but I fear that the Big Sur upgrade (ya right ) has messed with the fussion drive.
mmerges
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