That, I fully understand. I think many developers are productive in one language: in the one that they know best. Which is probably the one they use most. It might happen that this is is a "fast" language by accident (like Java or Go), or a language like Python. And then there is never enough reason to switch.

> "Most engineers would kill for a 5% speedup"

I think this is very rare - maybe a heavily used app in Facebook or Google, where 5% could mean a lot of money. But a factor of 10 speedup is much more common (and possible sometimes).

> there is an allure to performance optimization due to the fact that it can be so easily quantified.

That's true. I also think simplicity is quantifiable, and so my personal hobby is to write something impressive in few lines. Like a chess engine, QR code reader, editor, data compression tool, compiler, in 500 lines. But this is mostly for hobbies I guess. For work, it's mostly about features, and then performance, I guess.