PC or any other computer hardware doesn't get slower over time unless you are dealing with overheating issues which will lead to throttling.
Sure, a Windows refresh may be needed at some point but Apple products have the same issue.
PC OS has an issue, it is usually pretty easily fixable except in extreme cases.
Apple OS has an issue ... Apple tech support - "Have you tried wiping it yet?"
As for different system performance. Do you have links to tests run with the 2020 iPad Air 4 and the 2018 Macbook Pro and newer (same apps being benched are a must as is information about what platform each app it is optimized for).
Sucky performance on one platform is not necessarily indicative of the hardware's performance which is why I want data on which hardware platform said apps are optimized for.
I am not an Apple products guru so feel free to prove me wrong. At the same time I prefer PCs over Apple products. For what I use a computer for, a PC has always been way better than anything Apple.
Been building and repairing PCs since around 1991. I've also done some repair and upgrade work on Apple products in the last few years and while some stuff is nice, it is way more difficult to resolve when there is an OS issue compared to Windows unless you just do an OS reset and start from scratch.