Well, unittest faster. Don't check which exact part of it.
https://groups.google.com/forum/?hl=ru#!topic/pypng-users/T4dDTnuKlLI

May me delta will be smaller with removing this conversion in array mode, but there is also potential to reduce number of conversions. e.g. move them to higher level  (asDirect, asFloat etc.) where conversion also often used.