You could speed things up by trimming the PNG to miss out all of the white-space around its edges - then use it...
Use something like Gimp or PhotoShop.
Every transparent pixel has to be examined, so the fewer the better...
Also the front trimmed image gas relatively few 'holes', but the top view has hundreds !
Again finding all of the holes etc takes time.
You can also speed things up by down-sizing the images.
For example - the top-view is 640 x 640 [409600 pixels], reducing it to say 450 x 450 means there are under half the pixels to process, and not a lot of sharpness is lost.
Incidentally - on its own, trimming that image to remove the perimeter white-space removes 50800 pixels from the checking process [about 1/8th fewer pixels]. Reducing the image size to about half of the total makes additional savings...
So doing some image tweaking before trying to trim them...