the jpeg algorithms used in most codecs are sensitive to data rate of the movie-
the more information in a frame - the higher the data rate - the longer it takes to decode.
black and white movies usually have lower data rates - so can be decompressed faster
each frame in a movie ( in most movie files ) has different amounts of information - and decompress time varies throughout movie playback-
so its not dependant on the weather - its dependant on the movie data rates - and this varies on the type of images.
and everyones images are different.