We're using RS415 with FW 5.8.14.0. librealsense 2.8.3, Windows 10 1709.
When using one RS415 we're seeing low latency, but when we connect 3 cameras we see a very obvious increase in latency. We can reproduce the issue in the multi-cam example app.
I'm attaching links to 2 videos showing the issue:
one camera: https://drive.google.com/open?id=1eTAxBEujdxrIbn6UgkqmwUVRpPiCoJNx
three cameras: https://drive.google.com/open?id=1Aqve0U4FBlgJPOMnHyQRbiwyz5tRMMor

We also ran an experiment where we run each camera in a separate process at the same time, and the latency was low, just like in one cameras case.
We have a dedicated USB controller per cameras.

We are seeing increased latency with DS430 when running 3 in parallel as well.