RAM is specified by the fastest speed at which it can run. That's because it takes a finite amount of time to produce that data after it is given an address. If the RAM is too slow, the processor will read the data lines before the data has been made available, which is bad. If you put fast RAM in a slower machine, the RAM will make the data available just as fast as it always does, but it will actually be ready to read before the processor is able to read it. But that causes no harm at all. The RAM doesn't actually slow down in a slower machine.