I can see a PC benchmark sheet here.
It shows that the fastest XT is Juko XT, which runs at 12 MHz with NEC V20. I actually own one, and I've checked that it runs at 12 MHz. But another thing I know is that NEC V20 can operate even at 15 MHz, though I haven't found a computer that runs at the MHz.
What is the fastest 8088-compatible machine? If it's right that Juko XT is the fastest 8088-compatible machine, please also explain why NEC V20 was used to operate only up to 12 MHz on IBM PC-compatible computers.