Hi,
Some devices have data sheets that give some useful information, but now that you mention it i was thinking briefly about this a while back too where i wanted to use a light dependent resistor and laser to measure fan speed. It worked just fine, but i never looked into what the limit of the fan speed would be due to the LDR. I know they are slower than other types, but how much slower im not sure and never bothered to measure. Perhaps i should do this or look for some better data sheets. But sometimes i also like to use surplus parts that dont come with data sheets or even part numbers, so i guess a test or two is in order.