Even then, you're not going to be running on a wildly different set of architectures, I'm betting; it will be a finite set, of course, so you can attend to all

Even then, you're not going to be running on a wildly different set of architectures, I'm betting; it will be a finite set, of course, so you can attend to all the common optimizations and give the virtual machine what it needs to optimize the best it can. No one is going to write a single piece of code that runs on both a z80 and a 56-core Xeon, for example.