It's between 2 and 3.5x faster.
It's 9% slower on short string (10 bytes for the output), but I consider that the speedup is more interesting than the slowdown on short strings.
RMicrobenchmark:
Common platform:
Platform: Linux-4.1.6-200.fc22.x86_64-x86_64-with-fedora-22-Twenty_Two
Timer: time.perf_counter
CFLAGS: -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
Python unicode implementation: PEP 393
Timer info: namespace(adjustable=False, implementation='clock_gettime(CLOCK_MONOTONIC)', monotonic=True, resolution=1e-09)
CPU model: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Bits: int=32, long=64, long long=64, size_t=64, void*=64
Platform of campaign orig:
SCM: hg revision=90e41d965228 tag=tip branch=default date="2015-10-14 10:10 +0200"
Python version: 3.6.0a0 (default:90e41d965228, Oct 14 2015, 10:46:50) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]
Date: 2015-10-14 10:47:05
Timer precision: 54 ns
Platform of campaign optim:
SCM: hg revision=90e41d965228+ tag=tip branch=default date="2015-10-14 10:10 +0200"
Python version: 3.6.0a0 (default:90e41d965228+, Oct 14 2015, 11:07:24) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]
Date: 2015-10-14 11:09:53
Timer precision: 62 ns
-----------------------------------------+-------------+---------------
without spaces | orig | optim
-----------------------------------------+-------------+---------------
data = "AB" * 10; bytes.fromhex(data) | 167 ns (*) | 181 ns (+9%)
data = "AB" * 100; bytes.fromhex(data) | 621 ns (*) | 295 ns (-52%)
data = "AB" * 10**3; bytes.fromhex(data) | 5.15 us (*) | 1.65 us (-68%)
data = "AB" * 10**5; bytes.fromhex(data) | 500 us (*) | 147 us (-71%)
-----------------------------------------+-------------+---------------
Total | 506 us (*) | 149 us (-70%)
-----------------------------------------+-------------+---------------
---------------------------------------------------+-------------+---------------
with 0.5 space | orig | optim
---------------------------------------------------+-------------+---------------
data = "ABAB " * (10 // 2); bytes.fromhex(data) | 179 ns (*) | 186 ns
data = "ABAB " * (100 // 2); bytes.fromhex(data) | 659 ns (*) | 340 ns (-48%)
data = "ABAB " * (10**3 // 2); bytes.fromhex(data) | 5.48 us (*) | 2.19 us (-60%)
data = "ABAB " * (10**5 // 2); bytes.fromhex(data) | 529 us (*) | 194 us (-63%)
---------------------------------------------------+-------------+---------------
Total | 536 us (*) | 196 us (-63%)
---------------------------------------------------+-------------+---------------
------------------------------------------+-------------+---------------
with 1 space | orig | optim
------------------------------------------+-------------+---------------
data = "AB " * 10; bytes.fromhex(data) | 180 ns (*) | 191 ns (+6%)
data = "AB " * 100; bytes.fromhex(data) | 710 ns (*) | 330 ns (-54%)
data = "AB " * 10**3; bytes.fromhex(data) | 5.77 us (*) | 1.99 us (-66%)
data = "AB " * 10**5; bytes.fromhex(data) | 559 us (*) | 177 us (-68%)
------------------------------------------+-------------+---------------
Total | 565 us (*) | 179 us (-68%)
------------------------------------------+-------------+---------------
---------------+-------------+--------------
Summary | orig | optim
---------------+-------------+--------------
without spaces | 506 us (*) | 149 us (-70%)
with 0.5 space | 536 us (*) | 196 us (-63%)
with 1 space | 565 us (*) | 179 us (-68%)
---------------+-------------+--------------
Total | 1.61 ms (*) | 525 us (-67%)
---------------+-------------+--------------
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