I don't think I've ever had a Mac start to feel unusable like some of the iOS devices on the ragged edge of the last supported OS. I don't think I've had an iOS device start to feel like this recently either.
I think that's an opinion formed from the older devices limited hardware. I doubt any current hardware will feel slow for a long time... they are generally over specced for everyday tasks now and you need to find some edge cases for them being slow. Also apple silicon vs intel has nothing to do with this, it's all on the software side of things.
All the M1 cpus have the same single core performance, so the 'everyday' feel of using these Macs will feel the same over the whole range of Macs sold. I doubt Apple's going to intentionally cripple someone's $4000 M1 Ultra.
I have no concerns about my M1 MacBook Air being slow beyond it's 7 year label of "vintage" by Apple.