Difference in sending speed between 2 containers at one host and different host?

I’m setting up a Docker overlay network for 2 PCs, connecting by a router 100Mbps. In my case, there are 2 containers, one sends image and another receives image 640x480 RGB.

In the first case, I ran the two containers at only one PC.
In the second, I ran two containers at different PCs.

At the received container i measured the packet arrival time, it showed that in the first case the time is always less than the second case. Even if the bandwidth, which i measured using vnstat, used in both cases is much smaller than router’s maximum bandwidth.

Could anyone explain for me why this happened?