You can start Vim with a --startuptime <filename> flag, which will log every action Vim takes during startup into a file. For instance, here's how you write the startup log into startuptime.log:
$ vim --startuptime startuptime.log
Quit Vim, and open startuptime.log. You'll be greeted with something like this (I replaced sections of the file with <...> to make it easier to read):
In the preceding screenshot, you can see a set of timestamps (most in three columns), ...