pfSense

In some situations the cxgbe driver can introduce latency in 2.5.

It looks as though that is caused by a driver issue that has since been patched:
https://github.com/pfsense/FreeBSD-src/commit/42f7f5c5d22cc9e845e51a1f8a5b92573b8b3d2f

Typically users are seeing:

PING 10.100.10.12 (10.100.10.12) from 10.0.0.2: 56 data bytes
64 bytes from 10.100.10.12: icmp_seq=0 ttl=128 time=0.173 ms
64 bytes from 10.100.10.12: icmp_seq=1 ttl=128 time=20.169 ms
64 bytes from 10.100.10.12: icmp_seq=2 ttl=128 time=59.394 ms
64 bytes from 10.100.10.12: icmp_seq=3 ttl=128 time=0.161 ms
64 bytes from 10.100.10.12: icmp_seq=4 ttl=128 time=0.163 ms
64 bytes from 10.100.10.12: icmp_seq=5 ttl=128 time=0.206 ms
64 bytes from 10.100.10.12: icmp_seq=6 ttl=128 time=47.562 ms
64 bytes from 10.100.10.12: icmp_seq=7 ttl=128 time=0.169 ms
64 bytes from 10.100.10.12: icmp_seq=8 ttl=128 time=0.272 ms
64 bytes from 10.100.10.12: icmp_seq=9 ttl=128 time=0.158 ms 

That same test was all sub 1ms in 2.4.5p1