What resolution is your monitor?
Blue Iris isn't very efficient at drawing video to the screen, and this is especially noticeable at 4K resolution or if the scaling method is not set to "fast".
Although I think with sub streams working (as they are on your system) it shouldn't be bad anyway.
Open task manager to the Performance tab and monitor the CPU frequency. Under normal circumstances, the current speed should always be at or above the base speed. If it drops below base speed, that means it is throttling to save power, and this can cause CPU usage to appear artificially high. Or it can also happen when the CPU is overheating, though if the fans are working in the case and it isn't heavily coated in dust, that is unlikely to be happening.