The way you're doing it the descent into Cwm Glas Bach from Clogwyn is the quickest.
If you end up starting from the summit then it's supposed to be quicker to do Crib Goch out and back and then descend from Bwlch Goch into Cwm Uchaf and then Cwm Glas Mawr. This route avoids descending CG north ridge and is the one Colin Donnelly used on his record run.