I know this is late, but here is my analysis of the situation.
(((2 * 6) + (2 * 4) + (29 * 3)) * 15) = 1605
1605 is your megapixels per second (MP/s) with all cameras set to 15 FPS. It is really close to the limit of what an i7-8700K will handle, but it SHOULD be able to handle it if everything is configured properly as described in
Optimizing Blue Iris's CPU Usage | IP Cam Talk but it will probably be over 50% load and CPUs like the i7-8700K with hyperthreading go from 50% to 100% pretty quickly.
We are observing the CPU usage going upto 100% on web login and dropping as soon as logged out.. any thoughts?
This means you have turned on the "Limit decoding unless required" feature on most or all of the cameras, and that is the only reason your CPU usage is manageable. If you had been doing this two or three months ago I would have told you that you can't do this and still use the web server, because that "limit decoding" setting gets temporarily disabled for every camera being viewed through the web server and the first thing the web UI does is load the
All cameras group. You've seen the result of that.
Fortunately there is a fairly new option to let you run with it, located here:
"Require/decode all camera frames when streaming". Turn that off. Anyone relying on "limit decoding" to keep CPU usage under control should turn off the setting above for all their large camera groups.