Here is a patch for 3.4 based on Matt's patch with additional optimizations. It speeds up body_encode() and header_encode().

$ ./python -m timeit -s "from email.quoprimime import body_encode as encode; x = open('Lib/decimal.py').read()[:100000]"  "encode(x)"

Before patch: 1.12 sec per loop
After patch: 26.3 msec per loop

$ ./python -m timeit -s "from email.quoprimime import header_encode as encode; x = b'A'*100"  "encode(x)"

Before patch: 97.9 usec per loop
After patch: 23.7 usec per loop

For non-ascii data difference is even larger.