@KhMeRA I moved this thread to it own topic.
There are two reasons why this is happening. First, direct to disk mode MUST be enabled for all multimegapixel cameras. Direct to disk is not functional in demo mode. The cpu usage difference is huge.
Second, The i7-920 is and older power hungry processor and is not suitable for
blue iris. The 920 also requires a dedicated video card which adds to noise and power consumption and cannot take advantage of blue iris hardware acceleration that requires intel HD with quicksync.
A 300 haswell i5-4590 based optiplex or elitedesk pc will be way more powerful, WAY more efficient (will pay for itself in 1-2 years), and will be whisper quiet. Its worth the upgrade, it will pay for itself.