I assume the Pi broadcasts a WiFi hotspot and then they use some Remote Desktop software, but I'd love to know specifics of what people have found works!
Having said that, would also recommend the iSH app (https://ish.app/), which gives you an x86 Alpine Linux environment directly on the iPad without additional hardware.
Just to give an idea (and with all the caveats about this being a very unscientific comparison and not like for like in many ways), ‘time gcc hello.cpp’ runs in about .7s on that Pi* and 1.4s in iSH (on an Air Pad 2 from 2014). Using g++ instead, the time is 1.4s vs 1.8s.
So, yes, it would be crazy to use either for compute intensive tasks but they’re serviceable and not a million miles apart in terms of performance. The command line itself and apps like vim etc. do not feel ‘dog slow’ to me but there is likely an element of subjective ness there.
To be fair, I imprinted on using ANSI C on an Archimedes 310 with no hard disk where compiling would involve manually swapping 3.5” disks so this all feels like gravy...
* The first compile is actually also around 1.4s on the Pi then subsequent attempts come down to .7s, on iSH it’s more or less constant.