I recently encountered a situation where Vim was taking an excruciatingly long time to launch. From some manual testing, I was able to isolate the problem to a particular directory, but I wasn’t sure what was causing it. I then discovered Vim ships with profiling tools that would tell me how much time was spent in each of the functions that ran during startup. See the official docs for more info.

I’ll start with the shell command I ended up running:

vim -c 'profile start vim.log' -c 'profile func *' -c 'e config/routes.rb' -c 'q'

The -c option tells Vim to execute the given command after loading the first file (in this case the empty buffer), and will execute multiple -c options sequentially.

To break it down:

• -c 'profile start vim.log' starts the profiler, logging results to the vim.log file.
• -c 'profile func *' enables profiling of functions matching the given pattern. All functions, in this case.
• -c 'e config/routes.rb' opens the routes file. This was arbitrary for my purposes, as opening any file in the Rails application was slow.
• -c 'q' quits.

The vim.log file has a lot of detail about the functions that were executed. Here’s a short one that was near the top for me.

FUNCTION  fugitive#is_git_dir()
Called 18 times
Total time:   0.000396
 Self time:   0.000233

count  total (s)   self (s)
   18   0.000271   0.000108   let path = s:sub(a:path, '[\/]$', '') . '/'
   18              0.000107   return isdirectory(path.'objects') && isdirectory(path.'refs') && getfsize(path.'HEAD') > 10

That’s interesting, for sure, but the good stuff is at the bottom of the file, where functions are sorted by execution time. Here’s my top offender.

count  total (s)   self (s)  function
    1   2.203751   0.000457  rails#buffer_syntax()