OpenWrt Wi-Fi slow after idle: a 10-minute diagnostic

2026-10-11 · 2 min read

Symptom: after an idle period, one device's download drops to about 10 Mbps (sometimes worse) while other devices are fine. Here is the 10-minute diagnostic we used to find two root causes on an MT7986 router (grade E evidence; the fixes ship as scripts ^[2] and the measurements are published ^[1]).

1. Direction test: is it only the downlink?

# on the AP
iw dev <ap> station dump | grep -E "Station|tx bitrate|rx bitrate|signal:"

• tx bitrate low (MCS0) while rx bitrate is high means the AP-to-client direction is broken;
• if both are low, you have a signal/interference/regulatory problem instead.

2. Sample the rate, do not trust one reading

for i in $(seq 1 20); do
  iw dev <ap> station dump | grep -m1 "tx bitrate"
  sleep 2
done

Oscillating between MCS0 and MCS9+ points to rate-control/state problems. Pinned at a mid rate points to channel conditions.

3. Byte accounting: retransmissions or idle airtime?

Compare bytes the AP transmitted with bytes the client received during a LAN transfer:

• transmitted roughly equals received: airtime is idle, the rate is simply low (rate-control problem);
• transmitted far exceeds received: retransmission storm (radio/interference problem).

4. Check the two known mt76 traps

# trap 1: firmware VoW ATF (default 1) prevents the rate from recovering
cat /sys/kernel/debug/ieee80211/phy*/mt76/vow_atf
echo 0 > /sys/kernel/debug/ieee80211/phy*/mt76/vow_atf     # test it now

# trap 2: a 40 MHz-only client inside an 80 MHz BSS
iw dev <ap> info | grep -E "channel|width"                 # radio width
iw dev <ap> station dump | grep -m1 "tx bitrate"           # client width appears in the rate string

Expected result after fixing trap 1 (measured on MT7986): the first bursts are still slow, then throughput climbs back to about 400 Mbps within 15 s. If your client is a 40 MHz device, also switch the radio to 40 MHz (stable 330-420 Mbps).

5. Make the fix survive reboots

vow_atf lives in debugfs and is lost on driver reload, so persist it with an init script at boot (/etc/init.d/mt76-wifi-tune) and a watchdog that re-applies it every minute. Both scripts ship in the project toolkit.

6. Quick reference

Related: mt76 rate control deep dive · 80 vs 40 MHz · reproduction guide