Less latency when firewall is stressed

[dyonis0s]

Hello,

I made performance testing last time.
I configured OPNSense in bridge mode with 2 client.
Those clients forward their traffic through the firewall.
When I ping without stressing the firewall, I got about 1ms of latency.
But when I ping the firewall and generate a traffic to the maximum capacities of the firewall (about 1.5Gbps) my latency is two times less than not charged, about 0.5ms.
Any ideas of what is happening ?

Thank you

PS : I'm using Hyper-V 2012 R2