Most cameras are capable of multiple video streams. Stream 1 is usually at full resolution and high bit rates. The subsequent streams are at lower resolution and lower bit rates. What BI does if sub streams are used is to record continuously from the sub stream, which save significant amounts of disk space, and switch to the main stream for alerts so full detail, resolution, is available.
Using sub streams, in my case, allows for storage of about a month versus a week for full resolution only. It also unload the CPU significantly and will drop utilization very significantly as well, typically dropping by a factor of three to four. So, if your CPU is running at 50% using full resolution continuous recording switching to sub streams, continuous-triggered or continuou-alerts, will drop the CPU utilization to as low as 15% or so.