when you create a snapshot of the vm and disk, vmware creates a delta disk.
from now on, the new writes to the disk go all to this delta disk.
when you delete the snapshot, vmware tries to merge all the information on the delta disk to the original vm disk.
you said that the snapshot is from 2013 so it will have probabelly a LOT of data to merge. this is one of the reason because you shouldn't keep snapshots that long.
depending on the disk system performance this can keep up to hours...
do you have the possibility to look how big are your snapshots? maybe with RV tools or something simiar...