With i3wm, tmux and helix with clangd it is a capable development machine with a surprisingly modern feel, only the compile times give away that the machine is from the early 2010s. Even WiFi is working, and pkgsrc is fast and well-stocked with the newest software.
Like you said, the giveaway is the poor performance, but if you're a systems developer that usually isn't a problem anyway. Emacs, C, assembly, some Chisel and Forth are all that I write on it.
What do you think the 'p' in T42p really stands for? T42p did not exist when I bought my T42 back in 01 or 02. X13 keyboard is almost as enjoyable to use as the T42's was for the first decade of its existence, X13's wins hands down over a T42 keyboard with a decade of use and tired springs. I am pretty happy with their current keyboards.
It had 4gb ram, upgraded it to 8gb for $10 via Amazon, running Alpine with awesomewm and it works perfectly fine, firefox runs fine, compiling works fine, VLC works fine, etc.
Even Windows 10 ran on it fine. Really, CPU's haven't changed so much in the last 10 years, the focus has all bee on graphics.