Does an iPhone battery drain faster in cold?
Yes - run times are shorter when the battery is cold.

Apple has this to say: - https://www.apple.com/batteries/maximizing-performance/


  When using your device in a very cold environment, you may notice a decrease in battery life, but this condition is temporary. Once the battery’s temperature returns to its normal operating range, its performance will return to normal as well.


On a technical level, the drain is what the device uses and that doesn't change with the temperature being low. The ability of the battery to produce current and voltage is impaired only while it's cold. The charge is still in the battery, just can't flow as fast. Think molasses, but less dramatic in the cold.

To be even more precise, it's not physics like molasses, but it's the actual chemical reaction providing power slows down in the cold to deliver less than enough current to power the device.  

(Reference)