Observable profiles entities and tile entities and shows you what is taking up tick time and where, which makes it a practical tool for tracking down lag. The profiler GUI opens with the R key by default.
It has to be installed on the server to work; players without it can still connect but cannot use its features. It needs the Kotlin standard library (Kotlin for Forge or Fabric Language Kotlin). Available for Forge, Fabric, Quilt and NeoForge, Minecraft 1.16.5 - 1.21.4.
When your Minecraft server starts to lag, add Observable from the panel in one click and see the cause for yourself. 🔍