This Article provides information for enhancement done in vSphere 7.0U2 improving read I/O performance for VM running on  a snapshot (SE Sparse).

Snapshots preserve the state and data of a virtual machine at the time you take the snapshot. Snapshots are useful when you must revert repeatedly to the same virtual machine state, but you do not want to create multiple virtual machines. Virtual volumes snapshots serve many purposes. You can use them to create a quiesced copy for backup or archival purposes, or to create a test and rollback environment for applications. You can also use them to provision application images instantly.