Provide information on latency for Gitaly Cluster

doc/administration/gitaly/faq.md

@@ -24,8 +24,8 @@ Gitaly Cluster and [Geo](../geo/index.md) both provide redundancy. However the r
 The following table outlines the major differences between Gitaly Cluster and Geo:
 
 | Tool           | Nodes    | Locations | Latency tolerance                                                                                     | Failover                                                                    | Consistency                           | Provides redundancy for |
-|:---------------|:---------|:----------|:-------------------|:----------------------------------------------------------------------------|:-----------------------------------------|:------------------------|
-| Gitaly Cluster | Multiple | Single    | Approximately 1 ms | [Automatic](praefect.md#automatic-failover-and-primary-election-strategies) | [Strong](index.md#strong-consistency)    | Data storage in Git     |
+|:---------------|:---------|:----------|:------------------------------------------------------------------------------------------------------|:----------------------------------------------------------------------------|:--------------------------------------|:------------------------|
+| Gitaly Cluster | Multiple | Single    | [Less than 1 second, ideally single-digit milliseconds](praefect.md#network-latency-and-connectivity) | [Automatic](praefect.md#automatic-failover-and-primary-election-strategies) | [Strong](index.md#strong-consistency) | Data storage in Git     |
 | Geo            | Multiple | Multiple  | Up to one minute                                                                                      | [Manual](../geo/disaster_recovery/index.md)                                 | Eventual                              | Entire GitLab instance  |
 
 For more information, see:

doc/administration/gitaly/praefect.md

@@ -39,10 +39,26 @@ NOTE:
 If not set in GitLab, feature flags are read as false from the console and Praefect uses their
 default value. The default value depends on the GitLab version.
 
-### Network connectivity
+### Network latency and connectivity
 
-Gitaly Cluster [components](index.md#components) need to communicate with each other over many
-routes. Your firewall rules must allow the following for Gitaly Cluster to function properly:
+Network latency for Gitaly Cluster should ideally be measurable in single-digit milliseconds. Latency is particularly
+important for:
+
+- Gitaly node health checks. Nodes must be able to respond within 1 second.
+- Reference transactions that enforce [strong consistency](index.md#strong-consistency). Lower latencies mean Gitaly
+  nodes can agree on changes faster.
+
+Achieving acceptable latency between Gitaly nodes:
+
+- On physical networks generally means high bandwidth, single location connections.
+- On the cloud generally means within the same region, including allowing cross availability zone replication. These links
+  are designed for this type of synchronization. Latency of less than 2ms should be sufficient for Gitaly Cluster.
+
+If you can't provide low network latencies for replication (for example, between distant locations), consider Geo. For
+more information, see [How does Gitaly Cluster compare to Geo](faq.md#how-does-gitaly-cluster-compare-to-geo).
+
+Gitaly Cluster [components](index.md#components) communicate with each other over many routes. Your firewall rules must
+allow the following for Gitaly Cluster to function properly:
 
 | From                   | To                     | Default port | TLS port |
 |:-----------------------|:-----------------------|:-------------|:---------|