First I directly connected to the network. That means:
ONT > Cat6 > PC
After this my ping was around 0-1ms. No peaks. Average was 3ms.
Then added Asus RT-AC58U:
ONT > Cat6 > Asus RT-AC58U > Cat > PC
The pings are around 1-2ms but I see some peaks and not clear why.

I'm not a professional but I know that more layer/distance cause higher latency. Also quality means a lot. But I have well shielded cables and the distance between the devices is less than 2 metres.
So are those pings/peaks normal? What could cause those peaks if the PC just idle and this is the only connected device while I test? Do I have to care about these or not?