In this context, no it's not.

One thing that Code Jam does differently (and imho better) is that they give you an input set and you have 8 minutes (or so) to upload the solution. This is expected to give more than enough space for language speed variance.

Instead of execution time, coding time is much more precious in such competitions, and

Micro-optimizing has it's place, but it's usually not in a programming competition. that's what using ruby instead of C gives you leverage for that, as you can express more complex ideas and algorithms more easily (which should be the focus point).

This doesn't mean you shouldn't know the cost of operations.