As far as I can tell, VMWare is using copy-on-write logic to implement their snapshots. Therefore, when you create one, every operation done on your VM (eg. almost everything in runtime) would cause a little bit of the VM to be copied until the whole thing was essentially cloned.
Another performance issue with this is that reads would have to cascade to the original copy if the working copy doesn't yet have data (because nothing changed to cause a copy).
If you want to have the snapshots as a backup but can't tolerate a small performance decrease, consider cloning the VM instead.
