rootfs is also compressed on the go which actually makes all that blessed speed gain negligible if not counterproductive. If someone opened up rootfs he would be a saviour (in speed dept and a blasphemer as what is now in rootfs will need a LOT more space to fit. I can put 10mb of .c/.h files and they will take around 500kb of it. With uncompressed rootfs no libraries/dev packages that's for sure).
One easy experiment is moving microb binary to opt and comparing starttime after kill. For me opted binary starts faster (lots of non-plaintext code to decompress), but probably biased. See for yourself, difference in size taken from rootfs vs claimed is easily confirmed, just copy a file there and compare. Since saving rootfs is the goal, crowding it with qt, no idea why