I agree with you that this is a noticeable effect, but I have a hard time figuring out why it is, especially on Macs with the sealed system volume (which should make the operating system files bit-for-bit identical between a freshly set up Mac and one you've been using for years). Certainly it could come from the accumulation of cruft in /Library and ~/Library, but you can clean that up too.

Yeah, same - I tried the M5 Pro randomly while waiting for my daughter to finish salivating over a Neo, and just got surprised by how quickly it opened apps and reacted to anything I did - ~1-1.5 seconds for Affinity Designer was amazing.

My MBP probably got affected by OS updates like other people are saying - opening apps and responsiveness definitely got worse, and felt like slightly deteriorating over time (but it might have been MacOS updates along the way).