Exploration is low risk/minimal reward
Trading is medium risk/high reward
Combat is high risk/medium reward
I'm not quite sure what everyone is trading to get 3 million credits in 8 minutes (I'm going to guess Imperial Slaves in a Cutter, but I'm not sure), but trading is definitely the easiest way to go, even if it is mind-numbing sometimes.
Exploration is not at all the fastest way to make credits. I took a trip to Sag A* and back, and I made ~40 million credits. That trip took me probably at least 20 hours, if not double that. That works out to about 2 million credits an hour, at the absolute best. At worst (and, what I think is more realistic), it'd be at less than a million credits an hour, which is terrible. I can make 2 mil in an hour in a Haz Rez in my Vulture. I can make about the same trading in my Python, maybe a bit more now that I've upgraded the FSD and I can do my loop in 1 jump instead of 2.
Don't get me wrong. I love exploration. It's a hell of a lot of fun and I'm probably going to go back out pretty soon after Horizons drops. It's just not the most profitable enterprise in the galaxy. I genuinely feel like exploration isn't something that you should do solely for the money. It's really something that you need to want to do, otherwise you're going to go crazy.