I do not see why you think this - CBM developers were magicians? You think gcc cannot compile efficient m68k binaries?
To be fair, there *are* parts of AROS that are definitively not optimized for speed and low memory usage, so we will need to work on that once it's up and running.
As an example, the console.device in real AmigaOS has optimizations that only scrolls a single bit plane if the visible text uses only pen 1. AROS doesn't do any bitplane masking yet because it doesn't make any sense on the chunky modes used on PC hardware.
Though many optimizations like that are easy to make once AROS runs on Amiga hardware (in this case, just needs to do a SetWrMask() when we know its safe).