Most NPN silicon phototransistors that I see (Everlight, Kingbright, etc.) have fairly poor rise and fall times, on the order of 10us.
The best one I've seen so far is the QT Brightek QSD8T120B with rise and fall times of 7 us. Manufacturers always specify these times given certain load current and resistance and collector voltage, and in this case they are 200uA, 100R and 5V respectively. 7us would allow for a best-case data transfer rate of about 71kbit/s.
What is the limiting factor here? Junction capacitance? Is there a different technology for IR detection that I should look for to get theoretical data rates of at least 500kbit/s?