I'm trying to improve the runtime of an R packet which utilizes the by function by(data, INDICES, FUN, ..., simplify = TRUE) frequently. Does a parallelisable version of this function exist? 
According to its documentation, by is a wrapper for tapply, which in turn could be replaced by sapply and split. In case no parallelisable version of by exists, do I assume correctly that unwrapping the function towards its parallelisable core function would be the way to go?
