I've tried to follow the answer from this topic but didn't work: Run a GUI without the desktop

I have startx running automatically when my raspberry boots thanks to raspi-config, but now I need one more thing:

When X starts, I do not want it to open the desktop (i.e., show desktop icons, the desktop bottom bar, etc...), but just want it to run chromium like this:

chromium --kiosk http://localhost/

Probably it will be a bit faster opening chromium without having to load the entire desktop.

How can I make startx behave like this?