I've noticed unusual lag while using iPhone or iPad connected to OpenWRT router (I'm not at this location that often, so I can't rule out other influences, but I'm quite certain it was better when I set the router up).

So I tried pinging both iDevices, and the output was along these lines:

Reply from 192.168.xx.yy: bytes=32 time=3029ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=1ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=839ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=5ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=877ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=2ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=915ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=1ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=963ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=1ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=1011ms TTL=64
Reply from 192.168.xx.yy: bytes=32 time=1ms TTL=64

Literally every second ping looks strangely useless. Is this a result of some setting I can / have to change, or something else going on?

The internet speed is fine as usual. The lag is also evident when connecting to other local devices (that's when I started being suspicious).