Seems about right. There is some latency with the input and output for the card to do its thing. 7 frames is actually better than I've seen on other tests. Most of the delay is the output. Only way to get faster output is to use the GPU's HDMI and vMix's fullscreen function and convert it to SDI if needed in SDI format. Even that wont be zero latency, but less.

For better performance you will need a hardware mixer/switcher and to cut the delay down even more gen lock for everything will help. You will always have at least one digital conversion inside the camera if you are projecting live to same room screens but close enough to not be to noticeable. If you want to improve things in vMix you could play with the settings in the performance page of vMix settings e.g Low latency capture, High Performance Input Output. Your graphics card is performing some processing on your Blackmagic inputs when they hit vMix AFAIK.