I've been having this issue too since getting my M1 MacBook Pro at the end of 2020. App store updates always get to 75% very quickly (I assume due to my high download speed) but then take a very long time for the last 25%. Every time there is an Xcode update, it gets to 75% in just a couple minutes, but then the last 25% takes hours. There is no feedback in the UI about what it is doing, it just shows the blue circular progress bar around the stop button. It happens every time without fail, this most recent time on macOS Monterey 12 Beta 8. I have the 16 GB RAM + 1 TB storage model, and I have 911 GB of free unused storage. Why is it that the fastest ever product from Apple has the slowest ever app install speeds?