For SBCL, we recommend that you create a custom core file with socket support and POSIX bindings included because those modules take the most time to load. To create such a core, execute the following steps:
shell$ sbcl
* (mapc 'require '(sb-bsd-sockets sb-posix sb-introspect sb-cltl2 asdf))
* (save-lisp-and-die "sbcl.core-for-slime")
After that, add something like this to your .emacs:
(setq slime-lisp-implementations
'((sbcl ("sbcl" "--core" "sbcl.core-for-slime"))))
For maximum startup speed you can include the Swank server directly in a core file. The disadvantage of this approach is that the setup is a bit more involved and that you need to create a new core file when you want to update SLIME or SBCL. The steps to execute are:
shell$ sbcl
* (load ".../slime/swank-loader.lisp")
* (swank-loader:dump-image "sbcl.core-with-swank")
Then add this to your .emacs:
(setq slime-lisp-implementations
'((sbcl ("sbcl" "--core" "sbcl.core-with-swank")
:init (lambda (port-file _)
(format "(swank:start-server %S)\n" port-file)))))
Similar setups should also work for other Lisp implementations.