Update docs on how to disable LSP (#5877)

## Description

To be merged after
FuelLabs/sway-vscode-plugin#180

## Checklist

- [ ] I have linked to any relevant issues.
- [ ] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [ ] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [ ] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [ ] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: Joshua Batty <joshpbatty@gmail.com>

docs/book/src/lsp/troubleshooting.md

@@ -26,6 +26,12 @@ Then, make sure you only have the most recent version of the LSP server running.
 pkill forc-lsp
 ```
 
+### Large projects
+
+Sway projects with ten or more Sway files are likely to have slower LSP performance. We are working on better support for large projects.
+
+In the meantime, if it's too slow, you can disable the LSP server entirely with the `sway-lsp.diagnostic.disableLsp` setting. The extension will still provide basic syntax highlighting, command palettes, as well as the Sway debugger, but all other language features will be disabled.
+
 ## Server Logs
 
 You can you enable verbose logging of the LSP server.