Can you explain why “editing simple text documents has become painfully slow on my 2013 Macbook Pro”? Because the process is instantaneous on my 2013 Chromebook Pixel running Linux, or on my even older Thinkpads. I certainly don’t have to replace my computers every few years. Is it a problem with MacOS or the software you’re using?
As an example, I have for work a HP ZBook with an i7-6820HQ that clocks at 2.7 GHz, and 32 Go of RAM. I have to have Outlook, Teams and Lync constantly open because of work rules. If I open a web browser, the fans start spinning and the whole system slows down. Maybe I'm just cursed with bad hardware, but in 15 years of using Windows I never had a smooth experience, I've always encountered small slowdown, stutters and things like that. Same thing on Linux.
Perhaps you're cursed with corporate security. My Macbook Air is a beast compared to the maxed out Macbook Pro 16" with forced corporate security policy one corporation gave me.
I’ve twice deleted Sophos for slowness. I’m now running Bitdefender on a 4GB 2012 Air, and performance is reasonable (so long as I restart Firefox daily).
I'm not familiar with the ZBooks, but if they're as thin as the EliteBooks and ProBooks, and the cooling is as much of a joke, I can't say I'm surprised, especially with a HQ CPU.
My ProBook with an i5-8250U spins up its fan for no reason while doing next to nothing on Linux with i3. And no, there's no "borken power management because Linux" issue, the battery actually lasts a long time (comparable to the official specs, which are presumably for Winows).
But curiosity got the best of me one day and I opened it up. The cooling system is an absolute joke. The heatsink is ridiculously small, my iPhone 7 probably has a bigger one.
Same story with an EliteBook something, with an i5-7xxxU CPU. I actually switched the EliteBook for the ProBook because I could add extra RAM.
However, I don't have any "slowdowns or stutters" with this machine, and I usually run it attached to an external 4k screen.
Mine is more than a inch thick, I'd say it's about an inch and half thick. The cooling system is probably a joke on mine too, it often reaches 80°C while being idle.
Could be worth a try: did you open it and blow some compressed air through the heat sink? I did this when my MacBook was some 5 years old for the first time, and it helped with temperature and the fans would stop revving up during normal use.
Because OS and core upgrades these days seem more focused on features than performance, I think, and playing compatibility catch up with one another because the industry expectation is that people keep buying better hardware. Even if you do the same thing with a given piece of enterprise software, if the runtime for that software is doing all sorts of things to run features that you do not use and cannot turn off, then you pretty much have to "keep running to stay in place"
Possibly he is running an antivirus program or two. Maybe editing files over the network. I know that I've experienced these types of things in a corporate environment and that can lead to slowness where it otherwise would not be.