At 100k baud the transitions come every 10µs, you want the switching to be on the order of a few µs only. MIDI was designed to use 31250 baud specifically to be able to work with standard phototransistor opto-couplers.
Phototransistors are typically slow because there's no base lead for stored charge to leak out of - though there are a few optocouplers that do bring out the base, so you can speed them up somewhat.
Yes 5ns is definitely fast enough for 100kbaud!