PC817 Switching Frequency

80 KHz is listed as the roll off frequency. This is where a sine wave oscillation gets attenuated by 3db from the anticipated amplitude. Now if only the Miller Effect base collector capacitance of the photo transistor is the limiting pole at this frequency then you can model this output as a single pole filter. Thus you may be able to exceed this frequency limitation with some clever use of gain, pre-emphasis and de-emphasis filtering and likely comparators to exceed this bandwidth limitation. However, by doing this you will run into batch and degradation problems over time. In simpler words, you can get faster pulse streams past this 3db limitation with the use of additional circuitry. The manufacturer will not guarantee how many of these opto-couplers will perform faster or how long they will do this.