If the earth's rotation were to speed up or slow down would it have a drastic effect on the life on the planet?
Probably would depend on the rate of slowing down or speeding up.
When the Earth first formed the average length of a day may have been as short as 15 hours. Over the past 4 billion years or so, the Earth has been slowly slowing down due to the gravitational attraction of the moon that slows us down.
However, in general, life evolves fairly slowly and most complex species have evolved over a certain day/night cycle. Some species have evolved to be active in the daylight - some at night. If a rotational speed changes slowly, species can adapt fairly easily and its no big deal. If the Earth happened to slow down or speed up quite suddenly, species would struggle to evolve and keep up to the new day/night timing. However, its highly unlikely that the Earth's rotation will speed up or slow down suddenly.