Wired with the correct resistor value you can read LDRs as digital input. Experiment a bit. >3V on the pin is reliably read as high; <1.5V is reliably read as low (assuming 5V Arduino).
They're indeed slow (0.2 second reaction time or so) - how fast is your object, and how long does it cover each sensor?
Double is an alias of float in regular Arduino (8-bit AVR processor). 4 bytes, 6-7 significant digits.