The software-implementation would not have to be a direct emulation of what the hardware does. You are working with the type-system of whatever sandboxed language you are running, and can make much more high-level decisions about what accesses would be legal or not, or how they should get translated, instead of having to go through table lookups on each and every memory access. If you trust the JIT or the compiler you can even make many of the decisions ahead of time, or hoist them outside of loops to virtually eliminate any overhead.

A lot has happened since Smalltalk.