My interest in Vim's start-up process started many months ago when I was trying to determine what files were processed by Vim and why. But more recently someone emailed me to ask about one of my Vim Tips and I dug into the issue. The easiest and best starting point to look at your own Vim startup process is simply to start (Neo)Vim and type
:scriptnames - which, when I'm editing an RST file, as now, runs to a truly impressive 167 scripts. Should you want more details, try this:
$ vim --startuptime vimstartup.txt
Note that the filename supplied on the command line should be a throw-away file as it's going to have timing data from Vim start-up appended to it. I have to admit that I don't know what units those are in - probably milliseconds.
This should be enough to go on, but if not ... the next - and extreme/tedious - option is the Vim debugger.
- Almost entirely based on https://stackoverflow.com/questions/10790559/how-to-see-vimrc-files-used-during-startup