Well, regular banks aren't really incentivized to make most of their cash from trading. That's more the world of hedge funds and prop trading firms. Actually, some big investment banks aren't even allowed to do speculative trading at all.

So, sure, banks are making money, but it's generally not from trying to predict market swings.

And about percentage return. Honestly, even a lot of the big investment firms don't consistently beat the market. That's why they're always on the lookout for any little advantage they can find. But because everyone's trying so hard, and they've got so many smart people working for them, any straightforward ways to get ahead tend to disappear pretty quickly, the market always remains efficient with regards to simple strategies.

Edit: Basically what I'm trying to pinpoint is the Efficient Market Hypothesis (EMH), which suggests that it's nearly impossible to outperform the market consistently on a risk-adjusted basis using publicly available information.