The Apple IIe was designed to be timing-compatible with the Apple II+ so that timing-depndent software (such games and copy-protection systems) and hardware designed for the Apple II and II+ would, for the most part, work in the IIe.
However, the Apple IIe used a (presumably more expensive) 6502A, rated for 2 MHz operation. Why did they do this? Can I substitute a 1 MHz 6502 for the 6502A?