Same **** on my bike. The bike barely slows when I pull the rear brake lever, even after truing my wheel and bringing the brake pads in real close. With the front brake, however, I can stop the bike ridiculously fast. I figure it's the crap $4 cable I got at walmart. Even though I sprayed the whole thing with teflon/wax lube, it still has a ton of friction.

I'll source a new cable eventually but I'm too cheap for now; I just paid tuition.