Depending on the storage technology, the snapshots are deleted synchronously or asynchronously. The latter is most of the time not a problem for the VM, because the deletion is spread over a longer timespan, whereare the first one is direct and therefore has the biggest impact on the performance of everything else on the storage if (and this is the second question) the snapshot references a lot of data that is to be deleted. It is very hard to predict how much load there will be on the system. Even just saying how big the snapshot is or how much space would be reclaimed can only be done by reading the data and the snapshot and compute the information then. Therefore you will not find this information anywhere directly.
