Matt Gantz wants to see how far behind we are:
Availability Groups are useful for creating Highly Available (HA) database systems, but that doesn’t mean they are entirely immune to performance problems. In busy systems, limitations in the infrastructure can introduce replication lag that is severe enough to affect database performance in ways that aren’t immediately obvious.
In SQL Server Availability Groups, the relevant difference between synchronous and asynchronous replicas comes down to how and when a transaction on the primary server is considered “committed”: Although it is easy to measure the lag between asynchronous replicas by using the dashboard in SSMS or by querying the DMVs (Dynamic Management Views), it takes more work to find the latency between synchronous replicas. This article explains how to measure that latency using internal performance counters, offering a simple technique for monitoring the cost of synchronous replication.
Click through to read the whole thing.