diff --git a/go/logic/throttler.go b/go/logic/throttler.go
index 1635b5e..41ece40 100644
--- a/go/logic/throttler.go
+++ b/go/logic/throttler.go
@@ -84,18 +84,30 @@ func (this *Throttler) parseChangelogHeartbeat(heartbeatValue string) (err error
 	}
 }
 
-// collectHeartbeat reads the latest changelog heartbeat value
-func (this *Throttler) collectHeartbeat() {
+// collectReplicationLag reads the latest changelog heartbeat value
+func (this *Throttler) collectReplicationLag() {
 	ticker := time.Tick(time.Duration(this.migrationContext.HeartbeatIntervalMilliseconds) * time.Millisecond)
 	for range ticker {
 		go func() error {
 			if atomic.LoadInt64(&this.migrationContext.CleanupImminentFlag) > 0 {
 				return nil
 			}
-			if heartbeatValue, err := this.inspector.readChangelogState("heartbeat"); err != nil {
-				return log.Errore(err)
+
+			if this.migrationContext.TestOnReplica || this.migrationContext.MigrateOnReplica {
+				// when running on replica, the heartbeat injection is also done on the replica.
+				// This means we will always get a good heartbeat value.
+				// When runnign on replica, we should instead check the `SHOW SLAVE STATUS` output.
+				if lag, err := mysql.GetReplicationLag(this.inspector.connectionConfig); err != nil {
+					return log.Errore(err)
+				} else {
+					atomic.StoreInt64(&this.migrationContext.CurrentLag, int64(lag))
+				}
 			} else {
-				this.parseChangelogHeartbeat(heartbeatValue)
+				if heartbeatValue, err := this.inspector.readChangelogState("heartbeat"); err != nil {
+					return log.Errore(err)
+				} else {
+					this.parseChangelogHeartbeat(heartbeatValue)
+				}
 			}
 			return nil
 		}()
@@ -114,6 +126,7 @@ func (this *Throttler) collectControlReplicasLag() {
 
 	readReplicaLag := func(connectionConfig *mysql.ConnectionConfig) (lag time.Duration, err error) {
 		dbUri := connectionConfig.GetDBUri("information_schema")
+
 		var heartbeatValue string
 		if db, _, err := sqlutils.GetDB(dbUri); err != nil {
 			return lag, err
@@ -272,7 +285,7 @@ func (this *Throttler) collectGeneralThrottleMetrics() error {
 // that may affect throttling. There are several components, all running independently,
 // that collect such metrics.
 func (this *Throttler) initiateThrottlerCollection(firstThrottlingCollected chan<- bool) {
-	go this.collectHeartbeat()
+	go this.collectReplicationLag()
 	go this.collectControlReplicasLag()
 
 	go func() {