In most vSAN stretched-cluster configurations, latency or RTT between sites hosting VM objects and the witness nodes should not be greater than 200 msec (100 msec one-way).
The latency to the witness is dependent on the number of objects in the cluster. VMware recommends that on vSAN stretched-cluster configurations up to 10+10+1, a latency of less than or equal to 200 milliseconds is acceptable. If possible, a latency of less than or equal to 100 milliseconds is preferred. For configurations that are greater than 10+10+1, VMware recommends a latency of less than or equal to 100 milliseconds is required.