I daily a 2015 MBP. I own other newer hardware as well (dev kit; Linux box; wife has a modern MacBook I’ve used extensively). It’s not slow.
I just don’t run any Chrome junk at all. I still hit 8 hours of battery life and never experience slowdowns - even with Xcode running 24/7.
If you avoid modern bloat, we’ve had fast computers for quite some time. I’m sure the M1 is still another league, and I’ll likely buy one just to come forward a few years, but otherwise any Mac from the past 5-7 years is still very, very usable IME.
Short of that keyboard issue, that is.
I mean, part of it is that I don't work at a big company that requires I transpile 100k LOC every time I want to see my work on screen any more while attending to 100 slack channels at the same time, but I'm typically running Goland, VSCode, Safari (what I use personally), and Firefox (better for testing my web code) at the same time and it's snappy. Maybe 2-second compile typescript or go compile times, no noticeable slowdowns switching apps etc.
My laptop: MacBook Pro (Retina, 15-inch, Late 2013) 2.3 GHz Quad-Core Intel Core i7 16 GB 1600 MHz DDR3
From what I understand, USB-C Macbooks that came after this magsafe generation had to CPU throttle earlier to deal with thermal issues, so despite better specs they couldn't get pushed without overheating. As well as heat/throttle issues if you try to charge it from the wrong USB-C port.
Work also gave me a 2015 Mac and now Linux is running on it. It's pretty fine and far from sluggish. No idea what the author is doing to have problems.