If performance is too bad, you can only use the shell for toy examples, which means there's no reason to have a real shell at all and you might as well go back to COMMAND.COM or equivalent. It's taking the idea of Unix-style scriptable shells and Improving the implementation to the point of unusability.

Simple tools with simple rules will outlast most of the code we'll all build. Picking things like grep, awk, and sed means your knowledge will be widely applicable going forward, and many people caring about their performance both backwards and forwards in time means your shell can be pretty fast.

It's also inhibited by typical corporate crapware, but with a pretty barebones/vanilla config, in PowerShell takes a full four to ten seconds just to open a new session on my work computer.

PowerShell isn't even fast enough for basic interactive usage, never mind batch processing.