Most microcontrollers have the ability to set the ports as inputs or outputs. If the port is intended to be used as an input, but it's misconfigured as an output, then it will present a low impedance to one of the supply rails, depending on whether it's set high or low. If the port is set to an output, with a logic level low, it will be connected to 0V, via a low impedance. If the 74HCT14's output is high, then a high current will flow into the MCU's output pin, causing excessive power consumption and possible damage to the port. If it's possible the port could be accidentally configured as an output, add a series resistor to limit the current to a safe level.
The opto-coupler will be the the bottleneck, speed wise, but it will certainly be fast enough for 100Hz, so don't worry about it.