When do we prefer isolator over optocoupler?
What you show and call a "isolator" is a optocoupler with digital output.  The term isolator refers to various means to isolate a signal such that there is no common connection.  A opto-coupler or opto-isolator (different words for the same thing) is only one method.

Opto-isolators can be broken into a few broad catagories.  The most common is a LED shining on a phototransistor.  As you found, these usually have a few &micro;s to to 10s of &micro;s propagation delay, particularly when turning off.

Another type uses a photodiode as the detector, with active circuitry to amplify the relatively small signal from the photodiode into a digital signal.  These can have less than 100 ns propagation delay.  Note however that they require separate power on the receiving side, and generally cost more.

As should be no surprise when you find two different things that address the same problem, each has its advantages and disadvantages.  Everything is a tradeoff.