Reverse biased photodiode gain resistor selection
Yes, it will slow down the response (assuming the load presented by your sense circuit is still "high" compared to 1 MΩ). As far as AC circuits go, the junction capacitance of the photodiode is in parallel with the resistor, forming a low pass filter.

Increasing the resistance may also increase the noise, as the Johnson noise of the resistor goes as 

$$\bar{v_n^2} = 4 k_B T R$$