Nim is probably the most easiest language I've picked up. It's a delight to program in Nim and the performance is stellar (beats hand coded C in many cases). It's a rare feat really.
But even with GC on, Nim is super fast. I use it in real time rendering and I honestly didn't see any significant performance loss. In fact, it made some routines much faster because the Nim compiler produced much better C code than I ever did.
This is what I experienced with GC enabled systems programming languages (Oberon family of languages) back in the late 90's, just with C++ as comparison on my case.
The main problem is the culture of micro-optimizing code without profiling it, that is so prevalent in those communities.
(Of course, deep down the stack is always a non-GCed language, or at least assembly, so if your GC language wins a benchmark, you can always write that same program + the GC runtime in a non-GC language... but that is a trivial observation. Nim compiles to C, so is it GC'ed or not?)