PLC Speeds and Microcontroller Speeds

It's not possible to say, because some PLCs can have high-speed digital input cards.

Maxim: "On a process plant, if it happens in less than 1 sec, it isn't real."

Design characteristics of the plc components or of the microcontroller determine their speed. Most microcontrollers are design for a particular application speed will be part of its design in what application it used. Plc use all ready available components in most cases so will change with the industry. They could be the same speed one could be faster then the other which depends on the design.

To many variables to tell. The fastest in either topology will be for the time critical process to be interrupt triggered. With a microcontroller you can easily select an appropriate platform and program accordingly. In a PLC system you may have to sequentially scan all inputs and thus not select a process priority. But this can be a moot point if one works on a 100 kHz clock and the other a 100 MHz clock.