What's causing a bad response time of a phototransistor at 100 Hz
The data sheet is for a phototransistor, and the 2 us time is for 10% to 90% rise with a 100 ohm load.   

Phototransistors are capacitive, and with a lower load resistance (such as a transimpedance
amplifier creates) you can get faster operation.   It should be possible to use this sensor with a good amplifier to meet the modest goal of keeping up with
an LED modulated source.







simulate this circuit &ndash; Schematic created using CircuitLab