If you are running SLIME with SBCL on macOS you may notice that the REPL output seems to experience a bit of delay when compared to other implementations like Clozure CL or ECL for example.

This has been bothering me for a while, but I had no idea how to fix this.

I was suspecting this is related somehow to threading in macOS, but had no clue where the actual problem was, so I’ve posted a question on /r/lisp about it.

Shortly after, @stassats has provided a patch, which I’ve tested and it appears to fix my REPL delay issues.

You can find the original patch here.

Here’s how to apply the patch, if you are experiencing the same issues.

The steps below have been tested on:

• GNU Emacs 27.2
• SLIME 20210504.2053
• SBCL 2.1.4

The steps below assume that you have installed SLIME using M-x package-install and your SLIME installation directory resides in ~/.emacs.d/elpa/slime-<version>.

If you have SLIME installed somewhere else instead (e.g. you are using the latest version from the Git repo), make sure to switch to the correct directory first.

cd ~/.emacs.d/elpa/slime-20210504.2053/
cp swank/sbcl.lisp swank/sbcl.lisp.orig
wget https://gist.githubusercontent.com/stassats/2ff288d821937309aecca2424a2214ac/raw/b688959bc38c1d94c79e65f4b17088d7550e8efe/slime.diff
patch < slime.diff

Start up your SLIME session and you should be good now.