what is performance like?
Good, it benchmarks about as fast as a 2017 Core i7 Macbook Pro, I figure I build Swift packages at about the same speed. It's no M1, but it's good enough and much more portable: I just slip it in my pocket.
Are you doing web dev on it?
No, my phone is way overpowered for that, 
and I don't do any web dev.
Or are you going cray, cray, and compiling binaries on it?
Building the full Swift toolchain means building most of LLVM, which takes hours. The only drawback is that it rarely overheats when building such giant projects and is connected to an external monitor, as it has no fan.