Encoding Speed / During Playback Slider bars

There are 2 sliders bars on the Render tab “ENCODING SPEED” & “DURING PLAYBACK” I think i have an idea about what they do but can you explain.
On occasions we have experienced some sluggishness on the system and have attempted to adjust these settings but haven’t seen much difference.

ENCODING SPEED:

The maximum speed in FPS that the system will encode while you are not playing. (Default Unlimited)

DURING PLAYBACK:

The maximum speed in FPS that the system will encode while you are playing. (Default Paused)