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I wrote a quick benchmark:
---
import pyperf
import random
gen = random.Random()
# gen = random.SystemRandom()
gen.seed(850779834)
if 1: #hasattr(gen, 'randbytes'):
 func = type(gen).randbytes
elif 0:
 def py_randbytes(gen, n):
 data = bytearray(n)
 i = 0
 while i < n:
 chunk = 4
 word = gen.getrandbits(32)
 word = word.to_bytes(4, 'big')
 chunk = min(n, 4)
 data[i:i+chunk] = word[:chunk]
 i += chunk
 return bytes(data)
 func = py_randbytes
else:
 def getrandbits_to_bytes(gen, n):
 return gen.getrandbits(n * 8).to_bytes(n, 'little')
 func = getrandbits_to_bytes
runner = pyperf.Runner()
for nbytes in (1, 4, 16, 1024, 1024 * 1024):
 runner.bench_func(f'randbytes({nbytes})', func, gen, nbytes)
---
Results on Linux using gcc -O3 (without LTO or PGO) using the C randbytes() implementation as the reference:
+--------------------+-------------+----------------------------------+-------------------------------+
| Benchmark | c_randbytes | py_randbytes | getrandbits_to_bytes |
+====================+=============+==================================+===============================+
| randbytes(1) | 71.4 ns | 1.04 us: 14.51x slower (+1351%) | 244 ns: 3.42x slower (+242%) |
+--------------------+-------------+----------------------------------+-------------------------------+
| randbytes(4) | 71.4 ns | 1.03 us: 14.48x slower (+1348%) | 261 ns: 3.66x slower (+266%) |
+--------------------+-------------+----------------------------------+-------------------------------+
| randbytes(16) | 81.9 ns | 3.07 us: 37.51x slower (+3651%) | 321 ns: 3.92x slower (+292%) |
+--------------------+-------------+----------------------------------+-------------------------------+
| randbytes(1024) | 1.05 us | 173 us: 165.41x slower (+16441%) | 3.66 us: 3.49x slower (+249%) |
+--------------------+-------------+----------------------------------+-------------------------------+
| randbytes(1048576) | 955 us | 187 ms: 196.30x slower (+19530%) | 4.37 ms: 4.58x slower (+358%) |
+--------------------+-------------+----------------------------------+-------------------------------+
* c_randbytes: PR 19527, randbytes() methods implemented in C
* py_randbytes: bytearray, getrandbits(), .to_bytes()
* getrandbits_to_bytes: Serhiy's implementation: gen.getrandbits(n * 8).to_bytes(n, 'little')
So well, the C randbytes() implementation is always the fastest.
random.SystemRandom().randbytes() (os.urandom(n)) performance using random.Random().randbytes() (Mersenne Twister) as a reference:
+--------------------+-------------+---------------------------------+
| Benchmark | c_randbytes | systemrandom |
+====================+=============+=================================+
| randbytes(1) | 71.4 ns | 994 ns: 13.93x slower (+1293%) |
+--------------------+-------------+---------------------------------+
| randbytes(4) | 71.4 ns | 1.04 us: 14.60x slower (+1360%) |
+--------------------+-------------+---------------------------------+
| randbytes(16) | 81.9 ns | 1.02 us: 12.49x slower (+1149%) |
+--------------------+-------------+---------------------------------+
| randbytes(1024) | 1.05 us | 6.22 us: 5.93x slower (+493%) |
+--------------------+-------------+---------------------------------+
| randbytes(1048576) | 955 us | 5.64 ms: 5.91x slower (+491%) |
+--------------------+-------------+---------------------------------+
os.urandom() is way slower than Mersenne Twister.
Well, that's not surprising: os.urandom() requires at least one syscall (getrandom() syscall on my Linux machine).