Look at and understand Figure 13a. This is telling how long it will take to switch the device on and off. The gate of a MOSFET is like a big capacitor, but this capacitor changes it's stripes as you try to charge and discharge it. That is why there is a plateau in the diagram.
Does that help?
A small point. Did you notice that the TLP250 is: "Not Recommended for New Designs"? This means the part is nearing it's end of life (EOL) and may soon be unavailable.