FYI - why less fps? Answer less storage and less compute requirements.
As noted by
@pozzello "
right, the FPS affects the bitrate (network transmission bandwidth), the CPU required to process the video for motion etc, and the storage on disk, whether recording only on motion or full-time.
Some cameras are also just more reliable at lower frame/bit rates, even tho they claim to support higher... "
and
@area651
"If someone wanted a system that recorded a dozen cameras, all at 30-40fps, all wired (obviously I'd expect), and all continuous recording (to ensure you never miss anything) then what would the setup be? I wonder how much that would cost in cameras and computer alone. "
Right now, 5/2020 - Dahua's OEM 8MP non-AI cameras are easily available at 25-30 fps. Similar priced ranged 8MP AI cameras are running about 15 fps - largely due to the processing power required for the AI algorithms.
So some numbers,
8MP camera = 4x resolution of a 1080P / 2MP camera. Thus you'd need significantly more processing power to handle a 8MP resolution video processing for Encode / Decode and motion / AI algorithm processing.
Lowered frame rates will reduce the compute burden.