Here's something to remember. Some great composers in the past wrote some crazy songs with very fast series of notes but very few people know those songs.
But almost everyone know the easy, slow, catchy melodies like Twinkle twinkle little star. :)
It's not about speed. Just emotions.
If its fast its fast, its ok. But it is not what is important.

Joe Satriani's song Surfing with the alien his a good exemple of "needed" speed to go with the emotion of the song. Its fast and it fits the mood. It really feels like we are surfing, filled with adrenaline.
That's one of the reason he's out there rocking for the world and
those thousands of crazy fast shredders you can find on youtube are sitting infront of a webcam.