As root user, to simulate lag on a Linux system

  tc qdisc add dev wlan1 root netem delay 300ms

To remove

  tc qdisc del dev wlan1 root


Example output:

  When the rule is added, pinging a PC on a LAN

64 bytes from 192.168.254.42: icmp_seq=93 ttl=255 time=306 ms
64 bytes from 192.168.254.42: icmp_seq=94 ttl=255 time=303 ms
64 bytes from 192.168.254.42: icmp_seq=95 ttl=255 time=303 ms
64 bytes from 192.168.254.42: icmp_seq=96 ttl=255 time=304 ms
64 bytes from 192.168.254.42: icmp_seq=97 ttl=255 time=304 ms

  When the rule is removed

64 bytes from 192.168.254.42: icmp_seq=98 ttl=255 time=5.69 ms
64 bytes from 192.168.254.42: icmp_seq=99 ttl=255 time=3.82 ms
64 bytes from 192.168.254.42: icmp_seq=100 ttl=255 time=4.89 ms
64 bytes from 192.168.254.42: icmp_seq=101 ttl=255 time=3.60 ms
64 bytes from 192.168.254.42: icmp_seq=102 ttl=255 time=5.08 ms
64 bytes from 192.168.254.42: icmp_seq=103 ttl=255 time=3.44 ms
64 bytes from 192.168.254.42: icmp_seq=104 ttl=255 time=5.40 ms