As the title suggests, if you are experiencing startup issues or slowdowns with Steam, as happened to me, the most likely culprit is Armoury Crate.

ASUS' Game First service, included in the suite, blocks the connection of steamwebhelper, causing Valve's app startup times to extend up to 5 minutes.

I recommend uninstalling the entire ASUS Armoury Crate suite using the official Armoury Crate Uninstall Tool[www.asus.com].

Once done, Steam will start in just a few seconds as usual, and your OS will thank you too.