If I load a 400 picture folder to a slide show about 40 pictures will end up out of order. All pictures are numbered in order. The pictures come from my Nikon D7100 camera and are shot as high res jpegs not raw. Some pictures are cropped but if they are not they go at 11 megs each.
If they are played back on the TV it is through a blue-ray player. If through the computer it is with a DVD player.
I have been resizing my pictures 10 at a time like you said so it takes along time to get 400 done so I haven't tried a new show yet of 400 but 10 worked ok. Do I like it? No but I'll do it to get a slide show.