Perspective Workstation delay loading second display

I'm setting up a client to use Perspective Workstation (8.1.39, same as the Gateway) with two monitors, and there is a consistent 15s delay on startup before the configured page displays on the non-Primary monitor.
The page on the configured "Primary" monitor loads quickly, but the other page shows the spinning orange 3d cube with the Ignition logo at the bottom left for 15s before it attempts to load. If I change the Page Configuration settings in Workstation to make Display 2 the Primary, then the delay occurs on monitor 1. Even happens when loading two copies of the same page. Connect Timeout set at default of 30s, and Failback is disabled. Launching in Kiosk or Windowed mode gives same behaviour.
I do see in the Gateway log that there are two incoming connections with a timestamp about a second apart, but the second page still takes another 15s after this is received at the Gateway.

This is on Windows 10 (22H2), Gateway is on a local gigabit network. Does anyone have any ideas on how to get rid of this startup delay?