Yes, the CPU is underpowered. You will have to make sacrifices to get it running smoothly.
You should try to keep total system CPU usage under 90% if at all possible, to keep things running smoothly. Under 80% or under 70% would be better, so that there is a little bit of breathing room for when you are remote viewing or when some other service on the system needs to use more than a little CPU.
Depending on your cameras, fenderman's suggestions may or may not yield enough improvement. Notice that direct to disc recording must be set individually for each camera.
If
blueiris.exe's CPU usage is still too high, then you may need to start sacrificing camera frame rates. To do this, go into each camera's web interface and reduce the frame rate
a lot. The higher the camera resolution, the lower you will need to set the frame rate. You might get away with 10FPS each if they are 1 megapixel cameras, otherwise if they are 2 megapixel you may have to use 6 FPS or less. You'll have to tinker to find the sweet spot.