Stack machines are constantly using the same absolute offsets into the stack when evaluating in a loop, or evaluating different branches of a big expression. In fact, one of the simplest register allocators you can use in an expression code generator is a stack; pop to allocate, push to deallocate, and spill if you run out. On a CPU, this will have benefits, but in a VM, it's just a relabeling scheme for the same memory slots a stack VM would use.

What you potentially gain with a register VM is reduced accounting overhead; a stack machine will be constantly adjusting the stack pointer on every operation. It's a tradeoff for less complexity at codegen time. It's swings and roundabouts in the lowlands of performance, though; no loop and switch VM will be super-fast.