import time

BIG = 10 ** 6

data = bytearray(b'A'* BIG + b'B' * BIG)
third = BIG // 3
data[third:2*third] = b'B' * third
data = bytes(data)

pattern1 = b"B" * (BIG - 1)
pattern2 = b"B" * BIG

t0 = time.perf_counter()
data.find(pattern1)
print(time.perf_counter() - t0)

t0 = time.perf_counter()
data.find(pattern2)
print(time.perf_counter() - t0)