An i7-7700
blue iris machine can do way more than 12x1080p cameras at 15fps....it can easily do 30 1080p cams....not sure where you are getting that low number from....
if you need more than 30 cams at very high frame rates you need a commercial solution and be prepared to pay for it...you gotta understand the blue iris user base and who the developer caters to...its home/small business...its not designed to compete with commercial solutions with 40+ cams...low frame rates are sufficient in most case use scenarios....spending time on integrating high power video cards is a complete waste as only a very small percentage of the user base have a need for this...those who do require a commercial solution ..
Just got footage today from a Wells Fargo indoor ATM vestibule....low frame rates, horrid sub vga image from the overview cam and essentially unusable footage from the ATM cam....you are ahead of the game if you have 1080p at 10fps...