Looking at the restrictions, it make sense. Some of the jpg covers are indeed progressive, so no wonder they don't appear. They don't seem to be up to date though, as mpc uses APEv2 tags yet the covers appear.
Anyway, I am currently benching the non-scaling version using mp3 so that I can compare it to the OF later on. The last bootloader you provided to test the boot hang issue had OF boot functionality removed from what I remember. Could you please give me a link to one that can boot the OF?
Also, about the screen flickering, if it's not too much work, why don't you implement the fix and I could then run a benchmark test to see how much of an impact it really has. Personally, if the difference is in minutes, then it's worth keeping the fix.
Took a while, but here are the first results (MP3 v0):
OF: ~21h +- 18 min
w/o scaling: ~20h +- 10 min
w/ scaling: ~25h +- 10 min
A plus of 25% is quite a lot in my book, thanks for your hard work!
edit2: added the OF times. I'm assuming the CPU is not scaling in bootloader yet, but I'm a bit confused to see the battery discharge when plugged and running rockbox w/scaling.