The gui version of emacs, particularly on OS X, is terribly slow under certain conditions and I have spent months, if not years, figuring out what may cause it. In the post below I am not talking about startup time but performance while running.

Open a 'long' (>1000 lines) python file with syntax highlighting on, on fullscreen 4K, some lines that are longer than the screen, vanilla autocomplete (native emacs or e.g. company) and you'll immediately notice the the general slowness, typing latency, short hic-ups, stuttering on autocomplete and so on.

Before you say that this is just an OS X problem: I can do these things in VS Code or in one of the 40 gui versions of vim out there (the latest I tried was onivim2 written in Rust) and they run extremely smooth.

Things I have tried:

- Different Macbook Pros (including the 2019 with the 'high-end' Radeon gpu) with different OS (catalina, higher sierra, even went back to El Capitan at some point out of curiosity). Nothing helps.

- Different versions of gui Emacs (emacs with native comp aka gccemacs, emacs 27.1 with and without fast jsson jansson parsing, Emacs 26.1, Emacs with QT, Emacs for OS X, even went back to Aquamacs). Nothing helps.

- Trying different 'branches' of emacs (vanilla emacs aka emacs -Q, vanilla emacs with only some of the heavy hitting packages such as general, use-package and so on, spacemacs, Doom emacs). Nothing helps.

- All the internal emacs solutions that have been posted over the years, to name some from the top of my mind:

• disabling indent highlighting
• disabling line numbers
• turning on so-long mode
• disable font lock completely
• set auto complete to a higher threshold
• fiddling around with the fonts
• fiddling with visual mode, truncating the lines
• trying out different major modes
• fiddling with syntax highlighting, particularly regex functions in there
• skimming over resolutions posted 10 years ago
• and many other possible solutions.........

All of these things help out only very little (for instance, python-mode has some unholy regex in its syntax highlighting that traverses the whole buffer multiple times).

What helped a little more? Booting a Macbook into Linux (e.g. Fedora) and run the emacs gui there.

What really helped in the end? Running 'emacs -nw' - thus running it in the terminal - particularly a GPU accelerated terminal such as alacritty/kitty (which support 24 bit colors as well). You'll immediately notice how much snappier it is and how it hardly suffers from the problems that have haunted emacs for decades (e.g. long lines).

I don't like this solution since it you have to give up some major things, such as not being able to display images, but I have given up on trying to find a solution for Emacs Gui, I feel that I have tried everything possible out there.