SAME PROBLEM I have latest iMac 27-inch, 2020.. it has 1tb flash drive (ssd) and 64gb ram. It was lightning speed in everything, booting up took literarily 6 sec. on BIG SUR until the recent update 11.4 (20F71).. it's all slow now !!! I suspect this new update might benefit the new m1 chip and create some issues with current intel chip processors.. hope they will read this and fix it with another update.. Even if you factory reset the imac and install a clean Big Sur it will probably be downloaded with this new update..
ChrisHeart
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