Lower FPS always helps, and I found that full-time recording bumped CPU usage almost as much as motion detect. If you're connected to BI with other PCs to monitor over a browser, that adds considerable CPU load as well. I saw a little difference on my system when I changed the encoder to Xvid, but it was higher CPU usage, not lower.
Here's what I'd do to figure out where your performance limit is:
- Disable all the cameras, and note the CPU usage.
- Enable them one by one, and note the change in CPU for each. I like to leave 10% free CPU minimum to avoid bogging down the system.
It's possible that one camera's causing more load than others, and this will let you know how many your current box will support, and how much more CPU power you might need.
You can also look at the bit rate for all the cams on your stats screen to see if any are doing anything weird, and this will show any clients connected as well.