Building a midi project with Arduino + scrap components (optocoupler and diode)
The PC705V's response time is specified for a load resistor of 100&nbsp;Ω, i.e., when the transistor is not saturated.

To output digital signals (VCC and ground), you need to saturate the transistor, and then it is just as slow as any other phototransistor optocoupler (see figure 10 in the datasheet).

300&nbsp;V is overkill for a 5&nbsp;V circuit, but it does not matter. (Similarly, all optocouplers have thousands of volts of isolation voltage, which is not needed to break grounds loops between MIDI devices.)

If you really want to use the PC705V, add another transistor so that the phototransistor does not need to saturate:





The diode protects against ESD. You can use any fast switching diode (Si or Schottky), or leave it out if you want to live dangerously.