[FFmpeg-user] ~50% performance difference overlaying static image with png series using -vf/-filter_complex

[d-fens]

hi,

i have a background png and a sequence of pngs with transparency which  
should overlay the background.

when i use

./ffmpeg -i %04d.png -vf "movie=bg.png [bg]; [in] format=rgb32, split  
[T1], fifo, [bg] overlay=0:0, [T2] overlay=0:0 [out]; [T1] fifo [T2]"  
out.mp4

(taken from  
http://stackoverflow.com/questions/10438713/overlay-animated-images-with-transparency-over-a-static-background-image-using-f)
i get arround 13fps for fullHD images, but using

./ffmpeg -loop 1 -i bg.png -i %04d.png -filter_complex  
overlay=shortest=1 out.mp4

i get around 7fps for the same images.
I guess the background image gets re-read here but thats just a guess;  
as the second, newer version looks much more clean and simple i wonder  
whats the way to get best performance in this scenario?

greets!