Minecraft wiki entry for Development_versions, associated text: /jfr
When enabled, begins profiling with Java FlightRecorder, as well as the following custom events:
minecraft.ServerTickTime: Sampling event exposing average server tick times in intervals of a second
minecraft.ChunkGeneration: Time taken to generate individual chunk stages
minecraft.PacketRead / minecraft.PacketSent: Network traffic
minecraft.WorldLoadFinishedEvent: Initial world loading duration
The profiling can be stopped in-game by using /jfr stop.