MixCast can be a fairly intensive process depending on your configuration and hardware, but this means there are also possibly a number of options to improve the situation. Note that nearly any involved PC hardware could be a bottleneck, including the GPU, CPU, RAM, Harddrive, and even USB buses. As you might imagine, more powerful hardware can often un-bottleneck the performance issues being experienced, but there are also settings that affect each system which may be able to be reduced to get to full realtime operation.
Your graphics card is an essential component in MixCast's operation for several subprocesses. Check in your Task Manager (with "More Details" selected) to see if your GPU usage is bumping against 100%. Note that different experiences will have different graphics performance profiles.
If you suspect your Graphics Card to be the issue, you can try setting the Output Resolution setting for your camera to a lower value. This should result in an improvement to GPU usage if it is the bottleneck (although you may need to lower it more than once to see an effect). As a secondary measure, you can try reducing the Input Resolution of your Input Device (or try disabling background removal methods) to see if it makes an impact.
Your CPU is also used for a significant amount of MixCast's operation. Check in your Task Manager (with "More Details" selected) to see if your CPU usage is bumping against 100%. Note that different experiences will have different graphics performance profiles.
If you suspect your CPU to be the issue and not tied to a particular title, you should experiment with the settings of your input device (webcam, etc) in the MixCast configuration app. If the issue exists even with no input device selected, you may have an internal error with MixCast being triggered (see Support if you think this is the case), or your issue could actually be coming from another source but is causing the CPU to cycle needlessly as well.
USB is valuable for both camera inputs and VR headsets, but can sometimes cause problems when both are operating together (and even at times individually). It can also be difficult to determine at times as connection strength and performance can vary over time. You may be experiencing USB performance issues if you start experiencing issues with either your VR headset or viewing camera video when you have both active, for example in the MixCast configuration app (headset should be worn).
If you suspect this is the case, try distributing your connections to cameras and VR equipment across multiple groups of USB ports if possible. Ensure you're using USB 3.0 ports where recommended. Cameras can sometimes have physical connection issues, and should have their cable connections unplugged and replugged/flipped if possible as a test as well. If this doesn't resolve the issue you may want to purchase and install a USB expansion card, which attaches to your motherboard via PCI-e slot.