ccaru
Young grasshopper
I may have resolved the issue...
There was a single camera with a different Frame Rate (30). In the (actual, not Blue Iris) Camera settings I changed this to 15 like the rest, and everything came back to normal.
I don't think that the issue was native to the frame rate itself, but rather, the fact that there was a difference in frame rate between one and the other cameras.
No I haven't quite

Reading the manual, it appears that the fact that BI automatically adjusts the frame rate is intended behaviour, but then what can i do to fix my problem?
Basically, I set all my cameras up as 15FPS - CPU hovers around the 27% mark
After some time, the CPU levels skyrocket again - and i find that the Bi automatically set this rate to different values for each camera. The only way i found to resolve the CPU issue is to set these back all to 15FPS but then the cycle repeats itself.