RStudio "build source package" is very slow

If I run "build source package" in RStudio, I get output like this:

==> devtools::build()

✔  checking for file ‘/Users/david/huxtable/DESCRIPTION’ (583ms)
─  preparing ‘huxtable’: (4m 0.5s)
✔  checking DESCRIPTION meta-information ...
─  installing the package to build vignettes (650ms)
✔  creating vignettes (19.1s)

It's taking 4 minutes to "prepare huxtable". That seems crazily long. Does anyone know what it is doing and why it might be so slow? There's no compiled code in there.

It is much faster if I run devtools::build() on the command line, or if I run "R CMD build ." My script doesn't emit any errors or compile any code, so I am not sure this is the same as Why is building my package taking so long in RStudio? .