A 20C discharge rate means that battery claims it can supply 100A @ 7.4V (740W) and be discharged in 3 minutes of operation. I don't have experience with RC batteries, but I'm a little skeptical. That's an awfully fast discharge rate, and a lot of power from a 282g battery.
In the RC hobbyist community it's fairly widely recognized that manufacturers inflate their C ratings. That said, these things can definitely put out a scary amount of power, and discharging batteries in 3-4 minutes of flight isn't unusual for some high performance RC aircraft.
That is absolutely normal in the high performance RC world. You even get 40C and higher rates cells. No point carrying around a heavier battery than necessary for the power required.