While there was plenty of overspec in the AT design, it was built around a CPU rated by the vendor for 6MHz operation. You don't want the support headaches whe

While there was plenty of overspec in the AT design, it was built around a CPU rated by the vendor for 6MHz operation. You don't want the support headaches when some local reseller cracks the case, swaps in faster crystal, and things mostly work. There were aftermarket speedup circuits which you could turn on after the POST check completed; I recall one with a knob offering like 8 different speeds.