Function (fastMRCA) returns the most recent common ancestor (node number) for a pair of taxa; or, in the case of fastHeight, the height above the root of the MRCA of a pair of taxa; or, in the case of fastDist, the patristic distance between a pair of taxa.
This function is mostly redundant with findMRCA (or findMRCA(...,type="height") in the case of fastHeight), but for very large trees will be considerably faster.
(Also see getMRCA in the ape package.)