Increased jitter from devices on LAN to PPPoE Wan

Hello everybody!
My device is a MT7621 ZBT WG2626 powered by OpenWrt 19.07.3.
My ISP uses PPPoE. Everything works flawlessy, using SQM on a 100/10 connection.
There is just one small "problem": when I ping an external address from a device in my LAN, I see some jitter that is not there if I ping the same host from the router itself.

For example (test was performed when there was no internet activity):

## from the router itself
PING ( 56 data bytes
64 bytes from seq=0 ttl=120 time=17.986 ms
64 bytes from seq=1 ttl=120 time=25.282 ms
64 bytes from seq=2 ttl=120 time=25.104 ms
64 bytes from seq=3 ttl=120 time=25.022 ms
64 bytes from seq=4 ttl=120 time=24.361 ms
64 bytes from seq=5 ttl=120 time=24.684 ms
64 bytes from seq=6 ttl=120 time=24.551 ms
64 bytes from seq=7 ttl=120 time=24.113 ms
64 bytes from seq=8 ttl=120 time=24.388 ms
64 bytes from seq=9 ttl=120 time=24.194 ms

--- ping statistics ---
10 packets transmitted, 10 packets received, 0% packet loss
round-trip min/avg/max = 17.986/23.968/25.282 ms
## from a wired device
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=119 time=20.6 ms
64 bytes from icmp_seq=2 ttl=119 time=27.1 ms
64 bytes from icmp_seq=3 ttl=119 time=15.7 ms
64 bytes from icmp_seq=4 ttl=119 time=23.7 ms
64 bytes from icmp_seq=5 ttl=119 time=9.30 ms
64 bytes from icmp_seq=6 ttl=119 time=17.5 ms
64 bytes from icmp_seq=7 ttl=119 time=33.3 ms
64 bytes from icmp_seq=8 ttl=119 time=12.5 ms
64 bytes from icmp_seq=9 ttl=119 time=20.5 ms
64 bytes from icmp_seq=10 ttl=119 time=8.79 ms

--- ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9016ms
rtt min/avg/max/mdev = 8.789/18.905/33.346/7.427 ms

Does anyone have a clue what causes this? I tried to disable SQM but nothing changes.
I know it's not a big deal (latency is always low), I just find it odd. It seems to me that the problem is linked somewhere to the PPPoE.
FWIW, I had the same problem with my previous router (a GL-Inet AR150M).
Thanks in advance!

Bump, any thoughts? :slight_smile: