Let me try to explain this behaviour. The point is that snapshot information is gathered through datastore tasks, while performance data is collected with a separate flow through VI API. Getting snapshot information requires full datastore scan, only this approach guarantees finding all hidden/orphaned snapshots which are not visible via Snapshot Manager, thus meaning not visible to vCenter Server either.
Please be aware that all datastore tasks are triggered one by one, this is done in order not to stress vCenter Server/ESX(i) hosts with intensive scan tasks. So this is expected to have some sort of a delay before snapshot information is updated and displayed on the TOPs tab.