I should have done this with my 2020 Intel Macbook Pro. Now it's basically unusable and spins rainbow wheels constantly, because the OS is apparently garbage for an Intel system.
I had the same experience as you. I bought a super-fast 10-core iMac 27” with 72 GB of RAM in 2020. It ran beautifully until the moment I “upgraded” to Tahoe, when it instantly became unusably slow.
This cost me half a day wiping the disk, restoring the original OS, and then carefully upgrading back to Sequoia. Now it works well again, but I am counting the days to when Linux is the only way to continue using this great piece of hardware.
Another anecdote: a few years ago I was offered a shiny new never-before-used iPad for free by a client (a telco). There was only one catch, I was told: it was a few generations old. It had sat in its box on a shelf unopened for several years. I thought it might be useful at least as an e-reader, but I soon realized why it was now useless: I could not find a single app in the App Store that would install on it because every app required a “newer OS” (which wasn’t available). Most websites also refused to talk to its old (non-upgradable) web browser. Well played, Apple.
