Reprojection slows down request processing where custom projections are being used, but no appropriate geotransformation for it is defined in the AISDatums.txt file.
Users should be aware of the issue in that the using a Datum transformation that is not defined in the AISDatums.txt can substantially (40x) slow down requests. The work around is to ensure that any Datum Transformation used is defined. Unfortunately there is no easy way to determine what datums are being used by clients.