It seems to me that the point of using readinto() is to avoid double-buffering (and the extra alloc/free that comes with read()). The slowness of using a small buffer size seems like only a natural and expected consequence. 

The trade-off of accommodating a small buffer size (by buffering behind the scenes anyways) would likely slow the more common cases which use a decent buffer size. I am wondering if an effort to accommodate both uses would be appropriate. Possibly by not double-buffering if readinto(b): len(b) > buffer_size/2 (arbitrary but seems feasible), and copying directly as the patch does now. Otherwise, fill the buffer up for subsequent reads and copy len(b) to user buffer. There is probably a good equilibrium for when it makes more/less sense to bypass the internal buffer.

Changing _bufferedreader_read_generic() would require some of the other helpers to be changed as well but it may be the way to go.

Let me know what you think of the above. I will experiment a bit. Unfortunately I am a bit busy this weekend but cannot wait to work on this again.