I think it is SR=ID4/CL, assuming CL is connected between Vout- and Vout+.
When the input is railed in one direction, you get the full drain current (ID4) going through either Q5 or Q6. When the input swings to the other rail, the same current switches to the other (Q6 or Q5). So it seems like the current available to charge/discharge CL is simply ID4
SL= I/CL.where CL is load capacitance. Now using this concept u can analyze the circuit and will get the above equations. In the differential amplifier the current get switch from one branch to the other so consider this also n u will get ur answer