There's a trade-off, but I seriously doubt it's anywhere near that. For one, if you could get super fast by ignoring safety, that would surely be found in consoles. Then, a good old 486 or early Pentiums have none of these issues, having a multiple of z80 speed without some decades of progress we had in the interim.
I meant you could execute every instruction with cryptographic check, i.e. AES check for every single step if this instruction could be executed. That would bring speeds under Z80 and will be overwhelmingly secure if protocol is correct. I hope nobody would try to add it to normal CPUs in the future, but security guys are sometimes too idealistic.
