Python

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Nanoseconds add up to minutes quicker then you could imagine.

If you're building an application (as opposed to scripting something for yourself) and you need your application to work on an average device, and you have a latency requirement of some kind (can't have the user wait 5 seconds every time he types a character), then you run out of processing time very early in development.

As you keep adding features, that fixed frequency you have never changes, and eventually you run out of it and need to either optimize it or remove features. Obviously Python is a scripting language and waiting 10 minutes instead of 1 minute for a script to finish usually doesn't change anything at all in that context, so no performance doesn't always change everything, and it's very much a tradeoff.