Happy to say that this issue has been fixed in the latest 2.5.1 snapshots:
https://redmine.pfsense.org/issues/11602
Before - 2.5.0:
--- X.X.X.X ping statistics ---
500 packets transmitted, 500 received, 0% packet loss, time 701ms
rtt min/avg/max/mdev = 0.175/25.373/109.791/27.343 ms, pipe 8
After - 2.5.1-RC:
--- X.X.X.X ping statistics ---
500 packets transmitted, 500 received, 0% packet loss, time 96ms
rtt min/avg/max/mdev = 0.181/0.267/0.365/0.040 ms
A big thank you to entire Netgate / pfSense team for addressing this so quickly.