AG Sync Lag

;WITH UpTime AS
            (
            SELECT DATEDIFF(SECOND,create_date,GETDATE()) [upTime_secs]
            FROM sys.databases
            WHERE name = 'tempdb'
            ),
    AG_Stats AS
            (
            SELECT AR.replica_server_name,
                   HARS.role_desc,
                   Db_name(DRS.database_id) [DBName],
                   CAST(DRS.log_send_queue_size AS DECIMAL(19,2)) log_send_queue_size_KB,
                   (CAST(perf.cntr_value AS DECIMAL(19,2)) / CAST(UpTime.upTime_secs AS DECIMAL(19,2))) / CAST(1024 AS DECIMAL(19,2)) [log_KB_flushed_per_sec]
            FROM   sys.dm_hadr_database_replica_states DRS
            INNER JOIN sys.availability_replicas AR ON DRS.replica_id = AR.replica_id
            INNER JOIN sys.dm_hadr_availability_replica_states HARS ON AR.group_id = HARS.group_id
                AND AR.replica_id = HARS.replica_id
            --I am calculating this as an average over the entire time that the instance has been online.
            --To capture a smaller, more recent window, you will need to:
            --1. Store the counter value.
            --2. Wait N seconds.
            --3. Recheck counter value.
            --4. Divide the difference between the two checks by N.
            INNER JOIN sys.dm_os_performance_counters perf ON perf.instance_name = Db_name(DRS.database_id)
                AND perf.counter_name like 'Log Bytes Flushed/sec%'
            CROSS APPLY UpTime
            ),
    Pri_CommitTime AS
            (
            SELECT  replica_server_name
                    , DBName
                    , [log_KB_flushed_per_sec]
            FROM    AG_Stats
            WHERE   role_desc = 'PRIMARY'
            ),
    Sec_CommitTime AS
            (
            SELECT  replica_server_name
                    , DBName
                    --Send queue will be NULL if secondary is not online and synchronizing
                    , log_send_queue_size_KB
            FROM    AG_Stats
            WHERE   role_desc = 'SECONDARY'
            )
SELECT p.replica_server_name [primary_replica]
    , p.[DBName] AS [DatabaseName]
    , s.replica_server_name [secondary_replica]
    ,s.log_send_queue_size_KB / p.[log_KB_flushed_per_sec] [Sync_Lag_Secs]
FROM Pri_CommitTime p
LEFT JOIN Sec_CommitTime s ON [s].[DBName] = [p].[DBName]