From MCU programmers, I know you can make even a microcontroller run around a Xeon if you know how you can squeeze every cycle of performance, and exploit particularly hard tasks to optimise.

Write a riddle for a CPU with 100% cache miss rate, confusing the prefetcher to clog the memory bus, and enforcing a synchronous memory access. Such thing is very likely to run literally with an MCU speed on an x86 PC CPU.