by Marian Stofka:
Standard optocoupler speed is limited mainly by the relatively slow response of the phototransistor. This Design Idea adds components to the LED drive side to speed things up.
R1 is the original LED resistor, as used before the extra circuitry was added. Here however, its value can be higher, as the turn-on speed is determined mainly by the added circuit. You can thus save power, and also drive the LED with a less powerful driver.
Optocoupler speed-up also reduces power consumption – [Link]