Let us think a bit theoretical.

The Zippy LiFePo4's are 4200mAh capacity. 2S means 2 cells in series =6.6V and because there is is only 1P, it's 4200mAh

6.6V x4.200A=27.72W fully charged. Adapting to the charge losses and inffeciencies, add 10-15%=30W
So, charging @ 4200mAh means you'll be charging a completely exhausted battery in 1 hour and 10 minutes.

Cheap chargers like the imax B6 are rated at 50W peak. So you can charge the cells with 50/6.6= 7,5ah , in about 40 minutes.
Balancing can add a lot of time if the cells are out of tune with each other.

Some batteries can accept higher charge rates, meaning they can be charged in 10-15 minutes, but then we're talking high end cells. Also, getting chargers to do that higher charge speed is not cheap.