He might be talking about having to implement a register allocator/file in order to interpret stack machines faster. Something you get from the compilers on register architectures like RISC-V, ARM and x86.

wasmtime uses cranelift, which has a complex register allocator. wasm3 uses a simple register file.

> In M3/Wasm, the stack machine model is translated into a more direct and efficient "register file" approach.

https://github.com/wasm3/wasm3/blob/main/docs/Interpreter.md