replace_interleave in Objects/bytesobject.c and Objects/bytearrayobject.c can be optimized for the special case where the interleaving byte string is a single character.

Here's some quick results from timeit showing that it's about three times faster for the special case.
* Before (cold start):
>>> timeit.timeit('(b"x" * 2000000).replace(b"", b".")', number=1000)
7.619218342995737
* After (cold start):
>>> timeit.timeit('(b"x" * 2000000).replace(b"", b".")', number=1000)
2.7605581780080684

For the non-special case, running timeit.timeit('(b"x" * 2000000).replace(b"", b".0")', number=10000) takes ~173 seconds on both versions.

I reviewed your patch on Rietveld (you should get an email notification).

Addresses review comments.

I wrote a microbenchmark with my benchmark.py tool.

The patch always make bytes.replace(b'', char) and bytearray.replace(b'', char) faster even for strings of 10 bytes, the speedup on string of 1000 bytes or more is very interesting, even I never used this Python instruction :-)

-------------+-------------+---------------
type bytes   |        orig |          patch
-------------+-------------+---------------
length=10    |  250 ns (*) |  211 ns (-15%)
length=10**3 | 4.67 us (*) | 1.07 us (-77%)
length=10**5 |  441 us (*) | 78.2 us (-82%)
-------------+-------------+---------------
Total        |  446 us (*) | 79.5 us (-82%)
-------------+-------------+---------------

---------------+-------------+---------------
type bytearray |        orig |          patch
---------------+-------------+---------------
length=10      |  266 ns (*) |  224 ns (-16%)
length=10**3   | 4.67 us (*) | 1.08 us (-77%)
length=10**5   |  441 us (*) | 78.3 us (-82%)
---------------+-------------+---------------
Total          |  446 us (*) | 79.6 us (-82%)
---------------+-------------+---------------

---------------+------------+---------------
Summary        |       orig |          patch
---------------+------------+---------------
type bytes     | 446 us (*) | 79.5 us (-82%)
type bytearray | 446 us (*) | 79.6 us (-82%)
---------------+------------+---------------
Total          | 892 us (*) |  159 us (-82%)
---------------+------------+---------------

New changeset 62e3b7af0697 by Victor Stinner in branch 'default':
Optimize bytes.replace(b'', b'.')
https://hg.python.org/cpython/rev/62e3b7af0697

I pushed my latest patches, thanks for your contribution Josh.

Is it worth to optimize this pretty rare special case?

> Is it worth to optimize this pretty rare special case?

There was a TODO in the code, so I guess that the author wanted to write specialized code for 1-char replacement. Since the patch is short (adds 8 lines of C code), I consider that it's ok to optimize it.