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This is a Storage-intensive operation and thus may need to be throttled via resync options to not cause IO contention to VMs in a Production cluster.
Enabling vSAN Encryption has to do a rolling reformat of all Disk-Groups - this automated process entails:
1. Migrates all data off the Disk-Groups.
2. Deletes the Disk-Groups.
3. <optional whether selected> Writes random data to all blocks of the devices:
4. Recreates the Disk-Groups with Encryption mechanisms enabled.
Preferably before doing this, you would be running on a ESXi 6.7 U3 or later build that automates a lot of the IO-fairness scheduling as opposed to have to throttling the resync (and also has vast improvements to resync performance via better queue-utilisation).