Uhm, Yeah… 
(That awkward moment when your entire question is in the title)
No, not innately. It will be if you’re not explicit with your typing and rely on it’s dynamic features though.
For all practical purposes, no. Unity compiles both C# and JS equally as well. There may be minor differences between them, because some things are easier to do in C# than in JS (and vice versa), but my point still stands.
Okay, thank you for the replies! (was wondering if switching to C# would give me any performance boost)