I think rushing to get Johnny was the worst advice I was ever given online. Yes, you get more Keanu Reeves, but, with the exception of the quest involving monks, that doesn't lead to any great additional insight. They're just a few quips and asides.
I played it once by just doing what I'd always do in an RPG -- meandering around and doing a handful of gigs to get the lay of the land and the vibe of the game. And i played it once the "get Keanu fast" way. The first gave me one of the most mesmerising game experiences I've ever had, right the way from Act 1 to the very end of the game. The second felt... Completely flat and uninvolving, like I was a tourist in someone else's story that didn't really have much to do with *me*.
Doing extra gigs during Act 1, exploring, taking your time instead of rushing to the heist, allows you to bed yourself in the world before things really start to happen and, importantly, to make it feel like you have actually spent time with certain characters in your life. Without that, what comes after doesn't have any emotional resonance.
You do miss some content by slowing down Act 1. You lose a bit of Johnny. And you certainly don't *gain* content by spending more time in Act 1. But you gain the proper narrative beats that make the story work. If it's rushed, you break the proper delivery of the story.
Honestly, I think by giving players the freedom to zoom through act 1, and to zoom through the main story, without giving players the option to be signposted to the much fuller experience of the game, CDPR made a major mistake. Players can assume that the fast way is how the game is *supposed* to be experienced (not helped by Vik's unhelpful post-Act 1 lines that suggest an urgency that isn't there), when almost everything that is impressive about Cyberpunk depends on *not* playing it that way.