But the mitigations in AWS' VM monitor might be a fixed cost--i.e. not disabled on AMD architectures or otherwise not disadvantaging Intel. For example, they might use techniques like page coloring as an alternative to cache flushing and prediction suppression, which wouldn't disadvantage Intel, might not impose any significant overhead at all, and might even improve performance in shared hosting environments.

Alternatively, maybe their support for AMD is nascent and optimizations for Intel absent on AMD.

There's really no substitute for benchmarking AWS' actual offerings.