diff --git a/tools/testing/selftests/net/mptcp/simult_flows.sh b/tools/testing/selftests/net/mptcp/simult_flows.sh
index f441ff7904fc..ffa13a957a36 100755
--- a/tools/testing/selftests/net/mptcp/simult_flows.sh
+++ b/tools/testing/selftests/net/mptcp/simult_flows.sh
@@ -12,6 +12,7 @@ timeout_test=$((timeout_poll * 2 + 1))
 test_cnt=1
 ret=0
 bail=0
+slack=50
 
 usage() {
 	echo "Usage: $0 [ -b ] [ -c ] [ -d ]"
@@ -52,6 +53,7 @@ setup()
 	cout=$(mktemp)
 	capout=$(mktemp)
 	size=$((2 * 2048 * 4096))
+
 	dd if=/dev/zero of=$small bs=4096 count=20 >/dev/null 2>&1
 	dd if=/dev/zero of=$large bs=4096 count=$((size / 4096)) >/dev/null 2>&1
 
@@ -104,6 +106,16 @@ setup()
 	ip -net "$ns3" route add default via dead:beef:3::2
 
 	ip netns exec "$ns3" ./pm_nl_ctl limits 1 1
+
+	# debug build can slow down measurably the test program
+	# we use quite tight time limit on the run-time, to ensure
+	# maximum B/W usage.
+	# Use kmemleak/lockdep/kasan/prove_locking presence as a rough
+	# estimate for this being a debug kernel and increase the
+	# maximum run-time accordingly. Observed run times for CI builds
+	# running selftests, including kbuild, were used to determine the
+	# amount of time to add.
+	grep -q ' kmemleak_init$\| lockdep_init$\| kasan_init$\| prove_locking$' /proc/kallsyms && slack=$((slack+550))
 }
 
 # $1: ns, $2: port
@@ -241,7 +253,7 @@ run_test()
 
 	# mptcp_connect will do some sleeps to allow the mp_join handshake
 	# completion (see mptcp_connect): 200ms on each side, add some slack
-	time=$((time + 450))
+	time=$((time + 400 + slack))
 
 	printf "%-60s" "$msg"
 	do_transfer $small $large $time