This is most likely a power-related issue I described here:
https://opensprinkler.com/forums/topic/ospi-only-works-when-connected-to-hdmi-output/#post-41925
Because RPi 3 draws a considerable amount of current, it can cause the PTC fuse to drop higher voltage than previous versions of RPi. As a result, if your WiFi dongle is sensitive to the voltage drop, its performance will degrade. One option is to try a different WiFi dongle, another option is to solder a wire across the PTC fuse to bypass it. A third option is to power RPi 3 directly using a microUSB cable.
Obviously since OSPi was designed and manufactured before RPi 3 was released, these things could not be foreseen.