The D's GC collection cycle does pause, but it does not run unless the GC is asked to do an allocation.

Hence, if you don't do GC allocations, or at least do not do them in your realtime loop, it will not pause.

D is not a GC language. It's a language with an optional GC. (It also does not have the code gen compromises that fully GC languages rely on, such as write gates.)