How to measure CSS parsing and rendering time in a browser?
Use the Developers Tools panel which is built in to Google Chrome. It's also similar to that of Firefox and Safari with obvious minor changes here and there. That will tell you when the file is being loaded and also how long it's taking to load up as well.
Take a look at the documentation for Chrome.
You can open the Chrome DevTools by using the Alt + Cmd + I on macOS or Ctrl + Shift + I on Windows. It's a very comprehensive document with all the info there.
In case someone asks for the Firefox version, it's called The Waterfall. See here for more information. Again, it's very thorough.