Answer: 6/8 time can be performed at any tempo, slow or fast. But when 6/8 is fast, as it usually is, then each measure has two beats, each of which has the value of a dotted quarter note, equal to three eighths.

As in "Pop, Goes the Weasel," with beats marked with numbers:

But if the 6/8 is slow, then often the beat will actually be counted in 8th notes, six beats to the measure. The notation, though, does not change.