num_processed_meshes: The engine generates geometric meshes from MapBlock data for drawing. This is the number of those meshes that finished generating in each frame.
mainloop_sleep: If the game runs at a faster rate than wanted_fps, a sleep is inserted into each frame after drawing in order to not consume excess resources; this is that sleep time in seconds.
mainloop_other: Time spent in each frame for everything else than drawing.
mainloop_dtime: Total time spent per frame (mainloop_other + mainloop_draw + mainloop_sleep); FPS = 1/this averaged.
mainloop_draw: Time spent in each frame for drawing (rendering).
client_received_packets: Number of received high-level protocol packets in each frame.  https://cloud.githubusercontent.com/assets/3192173/16911725/719d36dc-4ce0-11e6-821f-0515e4640647.png
This article is incomplete.