6/8 is a compound time signature which is treated differently in music theory than a simple time signature like 3/4. The best way to explain this difference is that compound time signatures use a tuple (three eighth notes) for each quarter note beat instead of duple (two eighth notes). 6/8 time is not 3/4 in double time.
To illustrate 6/8 time, you would start with 2/4 time as a basis to build the meter. If you take 2/4. time and add an eight note for each beat, the meter would be this...
ONE and TWO and ONE and TWO and.
6/8 time uses the same general structure of 2/4 time, but places three eighth notes in the span of each quarter note beat like this:
ONE e an TWO e an ONE e an TWO e an
However, since it's 6/8 time, we would count the eighth notes when verbalizing this. It would be the same rhythm as the above example, but just say the numbers one through six instead. I've written numbers out in caps to emphasize the down beats, and the numeric numbers as the other beats:
ONE 2 3 FOUR 5 6 ONE 2 3 FOUR 5 6
If 6/8 time is based on the 2/4 meter, 9/8 and 12/8 time are likewise tuple versions of 3/4 an 4/4.
ONE 2 3 FOUR 5 6 SEVEN 8 9
ONE 2 3 FOUR 5 6 SEVEN 8 9 TEN 11 12
Again, each written out capitalized word is equivalent to the duration of one quarter note.
The reason that the beat of a 6/8 song may seem too fast is because you are not accounting for the tupled eighth note nature of the beat, but rather treating the eighth notes as a duple per quarter note. Because compound time signatures use 3 eighth notes in the span of a quarter note instead of two, the beat sounds 50% faster than you may be expecting.
As an example, if we have our song in 120 BPM, we would expect 120 quarter notes per minute. That makes each quarter note a 1/2 second in duration. If we were to simply double 3/4 time by playing two eighth notes per beat, the time of each eighth note would be a 1/4 of a second. However, since compound time signatures like 6/8, 9/8, and 12/8 use dupled (three) eighth notes per quarter note, the duration of the eighth note is 1/3 of a second.
To resolve this music theory conundrum, you'll want to reduce the BPM of your song as measured in 3/4 time by a third. For instance, if your song is set to 120 BPM, you'll want to set it to 80 BPM instead to accommodate for the compound nature of the time signature.