You seem to have a very good computer - with proper settings such computers are able to handle up to 2000 cameras simultaneously.
How many cores does your processor have? 99% CPU usage - is it from all cores combined or just from one of your several cores? In the latter case, it's not so bad, actually - for example, only 25% for each core in quadro-core processor.
In case of extensive processor load, a great option would be
switching to MJPEG or H264 substream. I’d like to note that the preview doesn’t have to necessarily be in lower quality, it can be in excellent quality as well - then it just needs to be an MJPEG stream. As soon as it's set up, you'll notice the difference. Please come back to us if you need assistance with that - but let us your camera models and configuration:
resolution, stream format, fps.
Also, please go to the Task Manager and see what process gives more CPU load than the others.