Index Card RPG has revolutionized the way that I DM for my friends and online players in a fantasy setting. ICRPG really strips away the minutae and grinding simulation away from what you'd expect to do in, say, D&D and and streamlines the process with a rules system that is very easy to learn and understand. You can have a rank beginner learn everything they need to start playing in a matter of minutes. No flipping back and forth between pages. No useless information or items on your character sheet. ICRPG Core is pure function, and that is the beauty of it. When we were running D&D 5th Edition (which I still think is GREAT, but not as good as this...) I could maybe get one encounter per night if they were big. Now my gaming group can pound through an entire dungeon in a single game session, and everyone still feels awesome.
You can pick up the Core, Vol 1, 2 and 3 of ICRPG for less than the cost of a single D&D book.... so why not? Give it a shot. I promise that once you adopt the concepts in this book to your 5e games you'll start wondering why you don't just convert everyone to ICRPG.