The shared medium (frequency spectrum) is what can add latency. If a device wants to talk over Wifi but another device is transmitting it has to wait. This introduces (variable) latency, aka jitter.

Here's an anecdotal example for you, in practice with actual equipment:

1) Mac pro via ethernet to router:

    # ping -c 5 -S 192.168.1.88 192.168.1.1
    PING 192.168.1.1 (192.168.1.1) from 192.168.1.88: 56 data bytes
    64 bytes from 192.168.1.1: icmp_seq=0 ttl=64 time=0.413 ms
    64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.396 ms
    64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=0.417 ms
    64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=0.553 ms
    64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=0.514 ms

    5 packets transmitted, 5 packets received, 0.0% packet loss
    round-trip min/avg/max/stddev = 0.396/0.459/0.553/0.063 ms

2) Same machine via wifi over Unify AP to router:

    # ping -c 5 -S 192.168.1.72 192.168.1.1
    PING 192.168.1.1 (192.168.1.1) from 192.168.1.72: 56 data bytes
    64 bytes from 192.168.1.1: icmp_seq=0 ttl=64 time=2.992 ms
    64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=4.136 ms
    64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=1.873 ms
    64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=2.293 ms
    64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=2.552 ms

    5 packets transmitted, 5 packets received, 0.0% packet loss
    round-trip min/avg/max/stddev = 1.873/2.769/4.136/0.774 ms

That's an average of 2.3ms extra latency, or 6x higher.