My first thought was that your database has a problem. Then I saw you were using my Lubuntu setup. Wonder if it has something to do with you using s single core machine? Does the usage go down if you stop Zoneminder?
I've seen this when the cameras (rtsp) are off line/disconnected/not there/or some problem.
If you don't have a max framerate set for each monitor/camera the system races as fast as it can.
I got mine up to 1236 fps once... cool... not...
Solution, set a max frame rate about 10% higher than you expect from your camera stream. If the camera's won't be online for a while, disable the monitor in function section of the offending monitor/camera.