Sometimes it is good to daven fast because there is no time for foreign thoughts when you are davening fast. In fact, I saw in a halacha sefer that the custom of Gerrer Chassidim is b’dafka to daven fast.
The Kav Hayashar mentions that his father and his rebbeim did not daven S”E at length.
At the same time, to be able to daven at length is certainly a good thing, as discussed in Chabad Chassidus.