[Python] Huge time difference when reading a file inside class

Any suggestions where this huge time difference results from when reading the same large text file by the same loop from inside a class in Python.

import timeit
fn = "some-large-file.txt"
pos = 2434735976

class TestCase:
def __init__(self, filename):
self.filename = filename
self.content = ""

def read_file(self, start):
self.readstart = start

with open(self.filename, "r") as f:
f.seek(self.readstart)
line = f.readline()
while line:
self.content += line.strip()
line = f.readline()
if line.strip().startswith('>'): return

timeit_start = timeit.default_timer()
a = TestCase(fn)
a.read_file(pos)
print(len(a.content))
timeit_stop = timeit.default_timer()
print('Elapsed time: ', timeit_stop - timeit_start)


90338456

Elapsed time: 31628.955818721

timeit_start = timeit.default_timer()
s = ''
with open(fn, "r") as f:
f.seek(pos)
line = f.readline()
while line:
s += line.strip()
line = f.readline()
if line.strip().startswith('>'): break

print(len(s))
timeit_stop = timeit.default_timer()
print('Elapsed time: ', timeit_stop - timeit_start)


90338456

Elapsed time: 1.233782830000564

I use Jupyter with Python 3.8.10 and IPython 8.12.2.

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