What options exist for a for low-latency (<1ms) photosensor?
Look into reverse biased / avalanche detection of photo diodes, usually fed to a transimpednace amplifier, the response time can be nanoseconds, 

For a start on reverse biased, here is the general schematic, when the brightness changes, the current changes very sharply, and you get a fast voltage rise.



Image Source

Avalanche is when you need it to respond even faster / sharper, but has a longer recovery time, generally its similar to this, but the bias voltage of the op amp is increased until its just below the reverse breakdown voltage threshold, this decreases with small increases in light intensity.