depends on the workload and how they've done it

pure CPU should be essentially unaffected, if they're not emulating the MMU/page tables in software

the difference in IO ranges from barely measurable to absolutely horrible, depending on their implementation

traps/vmexits have another layer to pass through (and back)

From memory, the virtualisation operations themselves aren't nested. The VM instructions interact with the external virtualisation hardware, so it's more of a cooperative situation, e.g. a guest can create & manage virtualisation structures that are run alongside it.

I don't know if this applies to the specific nested virtualisation AWS are providing though.

As a practical matter, anywhere from 5-15%.