The 3 hours to charge from the mains is due to the fact that the battery can only absorb energy at a certain rate. You could get all the energy needed to charge that battery from the mains in a second or two if the battery could absorb it fast enough. In other words, a battery that takes 3 hours to charge from the mains might charge in 3 hours using a USB charger as well.

Check over on the electronic and gadget sub-forum. There are some very knowledgeable people over there that will be happy to help. Personally, I'd go with lithium ion 18650 batteries and a flashlight made for them. The only question is if you can find a charger for these batteries that will run off a bicycle generator.