[tadfisher]

Emacs was built as a TUI, and it is built like a giant REPL; read input, evaluate command, print the output, and loop back. IDEs based on "modern" GUI frameworks (IntelliJ uses Swing, not exactly "modern") are inherently multi-threaded; you have the UI (or "main") thread that accepts events and updates the display, and ideally never blocks, while evaluation is performed on worker threads that can be canceled or scheduled as needed.

In practice, you can get far with Emacs' process supervision to avoid blocking the command loop, but some things that are trivial to offload to a worker thread are next-to-impossible to avoid blocking in Emacs. One of those things is parsing megabytes of JSON from an LSP server into Lisp data structures, which is baked into the Emacs core and will block redisplay.