I wasn’t getting 30fps all the time. The fps would vary based on CPU load.
You may be able to contact the people at TwinCamGo to ask about their experience. They are using a relay to get the video to Unity. In their setup, the camera is not plugged into the same computer that is controlling the headset signal.
For your Windows setup, the driver from RICOH may not be working properly on your system. As you tried it on 4 systems, the problem could be related to either the Japanese version of Windows 10 or the Japanese version of Unity. Occasionally, some string in a program causes problems.
We have not been able to identify the source of the problem.