Once per second is a hardware limitation. Most GPS chips, until recently, only compute the position once per second. Chips which produce more readings per second are available, but would be bad in a phone, since they probably need more power.

Note also that GPS speed is not accurate at low speeds, particularly below 3 MPH or so. Since you are trying to measure acceleration you should be using the accelerometer, it has an update rate of 20Hz, and is very easy to read from python.