note on slow hooks

vignettes/FAQ.Rmd

@@ -42,11 +42,13 @@ Yes, all but the `open_config()` and `open_wordlist()` to open files in RStudio.
 
 ## Can I use the hooks provided in this package without installing the R package?
 
-Yes. The hooks work even if you don't have the R package installed 
+Yes, apart from the `roxygenize` hook (which uses an exported function from 
+{precommit}). The hooks work even if you don't have the R package installed 
 because all you need to have for the hooks to work is the pre-commit executable. 
 It will `git clone` the hooks from this repo into `.git/hooks` or similar. All
 exported functions of this package are just for the users convenience.
 
+
 ## How can I make sure that my contributors are using the hooks?
 
 They must follow the installation instructions in the README, i.e. run 
@@ -75,6 +77,16 @@ described [here](https://pre-commit.com/#temporarily-disabling-hooks).
 
 How to contribute new hooks is explained in [`CONTRIBUTING.md`](https://github.com/lorenzwalthert/precommit/blob/master/CONTRIBUTING.md).
 
+## Why are my hooks running slowly?
+
+Some of the hooks depend on R functions that run slowly, e.g. the `style-files` 
+hook depends on `styler`, which is not very fast. Also, note that for each hook,
+R is started. Make sure your startup process is quick for non-interactive use, 
+e.g. your `.Rprofile` and related files do not have a long execution time. E.g. 
+wrap code in `if (interactive()) ` if possible, so it won't run when pre-commit 
+hooks are ran. Find out more about the startup process with `?startup`. We'd 
+like to support the `--vanilla` option but it's 
+[not easily possible we believe](https://github.com/lorenzwalthert/precommit/issues/165).
 
 ***