What does buffer size do OBS?

Use Custom Buffer Size / Buffer Size x264 will encode everything into a fixed size buffer of this size. Raising it can increase motion quality, but at a cost; if there is a sudden scene change and high motion, it can fill up this buffer at a rate faster than the average max bitrate.

What is a good buffer size for streaming?

What is the best buffer size for streaming? To eliminate latency, lower your buffer size to 64 or 128. This will give your CPU little time to process the input and output signals, giving you no delay.