The Implementations box on the front page https://www.scheme.org/ has
been bugging me for a long time:

1) R5RS support is indicated inconsistently. It would be a good policy
to presume that all listed implementations (i.e. for which we have
subdomains) support R5RS. In other words, we won't list R4RS and R3RS
implementation (yes, there still are a handful of them around).

The Implementations tab https://get.scheme.org/ could have a more
comprehensive implementation gallery including even the exotic ones.

In practical terms, all major implementations support R6 and/or R7
except Bigloo.

2) We call implementations "fast" quite haphazardly. There's no
consistent metric. It's not based on benchmarks, and even those are
questionable. I suggest we don't call any implementations fast, and
instead draw attention to notably advanced optimizations where such exist.