If speed is of the essence, you will always do better with converting to a custom format. You might even save a few bytes in doing so! The fastest solution is to decode ilbm and save as raw files, then switch bitplane ptrs to that frame. By Grabthar's hammer... what a savings.
If this is for replaying animation frames, you will save even more space and time by making a player compatible with IFFanim.
But I think you just want to make the ultimate most fantastic superfast *drumroll* ILBM converter... which I find utterly unnecessary but there you go
Just REPT 256 the copy or fill instruction and jump into the chunk of moves at an offset of (256-count)*2. For the fill, move (a0) into Dn first.