Keep in mind that mzscheme is battle tested. Arc is "just" a bunch of macros on top of PLT Scheme; what gives HN its [under]performing character is that foundational JIT compiler and 3m GC.

Arc compiles into MzScheme, but it's not implemented as macros. You can see that from the source.

In general, 3m is performing very well, much better than the conservative collector plt used to use by default (the boehm gc). As for the jit: many of the benefit of the jit are irrelevant in Arc since it doesn't use mzscheme modules; this is in addition to using an old mzscheme version, when the jit was rather new -- many many improvements were done since then.