With generic optocouplers, the main concern is to improve CTR.
High CTR can be achieved with high emission LEDs and/or wide-base phototransistors. Usually the latter approach is followed.
Wide-base means relatively large capacitance, which will limit your switching speed. Therefore the vendors are reluctant to provide a firm, guaranteed switch time value.
If you want a guaranteed switching value, which also will not degrade significantly over time, you have to select an optocoupler which has been optimized for switching speed.