You need to clarify that you're talking about development speed, not runtime speed, since you're talking about C here!

At runtime, Ruby is as slow as a snail in the tundra.

In fact, I'd have put Ruby on the "don't learn" list. It has pretty much nothing going for it at this point. Python is as fast, almost as compact, has a bazillion more libraries, and is far more widely used (meaning your skills will be more widely applicable).

And definitely learn C over learning Ruby! C will teach you way more about how hardware really works, and will give you a way to write code which runs fast. (Or, better, like you said, pick Rust. But not Ruby! Never pick Ruby these days, except for historical reasons, or if it just absolutely clicks with you.)