I'm learning BI, so your previous system performance is of some interest to me.
The way I understand it, if the "direct-to-disc" feature was enabled on each camera, there really isn't much for the CPU to do while recording. As the computer receives video from the cameras, it writes it "direct-to-disc" .. no real compute happens. On the other hand, if you didn't have "direct-to-disc" enabled on some (or all) of the cameras, as the cameras send the video to the computer, the computer has to transcode each camera stream from whatever format the camera sends to whatever format
Blue Iris defaults to for transcoding, them write the video to disc. This adds significant load to the processor. This is what jumped to mind when you said your previous system couldn't record more than two cameras at a time...
Also, Blue Iris makes use of the Quick Sync feature on certain Intel processors to hardware accelerate H.264 video. A lot of Xeons do not support Quick Sync. So whenever the Blue Iris client is open and displaying video, not having Quick Sync means that the processor itself has to do all of the work. This also adds can add a significant load to the processor.
Interesting. I've got an old system (2012) running an i7-3770. I've got 7 cameras recording (15 MP at 20 FPS) with a total system CPU usage between 10-15% total. That's with direct-to-disc enabled on all cameras, and Blue Iris using hardware acceleration enabled for H.264.
If I get a chance in the next few days, I'll clone four of my cameras in BI. That will bring the total cameras up to 11, with 7 recording and 4 monitoring. Curious to see the impact on the CPU.
If you haven't seen this already, it's a great resource for tuning BI:
Optimizing Blue Iris's CPU Usage | IP Cam Talk