Spin-transfer induced magnetization switching in a MgO-based magnetic tunneling junction (MTJ) has been measured over a wide time range. It was found that the switching current response is asymmetric going from the high resistance state to the low resistance state and vice versa. This asymmetry must be taken into consideration to optimize the read-write margin for a 1T1R (single transistor, single MTJ) memory device design, especially since the driving current that the transistor can supply is also asymmetric. The MTJ and the transistor characteristics need to be adjusted so that the spin-transfer switching current asymmetry matches the transistor driving current asymmetry at the desired operating speed. A new device configuration is proposed to achieve this optimization.