I go for crossfit early morning. I usually have just a banana (or maybe half of it) and go for the workout. I don't really feel weak or anything. Sometimes i think i can go even without eating anything.
Do you advice to eat more than that or is it ok?
Short answer: no.
Long answer - do what makes you feel/perform better. I usually go into a WOD slightly hungry. Not starving, but hungry. That way my insulin is down and I'm pulling fat out for energy. That means I may not be as fast as I could be if I were purely running glycolitic, but for any workout over 5 minutes, I will win the day. Under 5 minutes, (e.g., Fran), I'll have some sugar before the workout if I'm trying to win on the whiteboard (but I do recognize that it's just for ego and not actually making me better).
If you feel good and are performing the way you want to, I don't see why there would be any reason to mess with your routine. Do what works for you! Personally, I prefer working out fasted whenever possible. I feel as though I perform better that way.
I think you just answered your own question.
If you're performing well and not feeling bad, your breakfast banana is enough. If you're underfed and overworked, you're guaranteed to feel like shit.
The fact that you don't feel like shit means you're neither underfed nor overworked.
I don't crossfit, but I do Olympic Weightlifting, round-robin the platform with 2 other lifters. I get about 2-6min rest between sets for about 1.5 hours... spending about 20 minutes warming up, so it's not "medium intensity" at all.
My best training days are when I get into the gym fueled, but hungry. This means that for breakfast I'll have a slightly bigger breakfast (extra eggs, some potatoes (I'm normally LC), fruit, veggies, and meat), then I'll skip lunch and train in the early afternoon.
Thanks to Art DeVany's influence I also avoid eating immediately post-workout as well. I have a 40-min drive home from the gym anyway so I end up training from say, 2pm to 4pm, and I make it home by 5, prepare a meal and eat by 6.
Back in the day I used to focus solely on pre/post workout nutrition. It didn't change my rate of progress or recovery, and that is why I don't even bother anymore. I also found that a big protein shake immediately after a brutal set of 20-rep squats or tireflips, usually would reverse direction fairly quick and I'd have to get a mop.
In a nutshell, I don't eat at least 3-4 hours before, and usually 1-2 hours after I train. This works well for me.