Ah, sorry, false alarm. "b[:] = b" actually makes a temporary copy of the bytearray when assigning to itself (!).

However, there's still another strange regression:

$ ./python -m timeit \
  -s "n=300000; f=open('10MB.bin', 'rb', buffering=0); b=bytearray(n)" \
  "f.seek(0);f.readinto(b)"

-> default branch:
10000 loops, best of 3: 43 usec per loop
-> default branch with patch reverted:
10000 loops, best of 3: 27.5 usec per loop

FileIO.readinto executes a single read() into the passed buffer.