import io, tarfile, time

def build(n, blk=4096):
    # GNU sparse 0.1 (pax) member: n data blocks separated by holes.
    t = tarfile.TarInfo("GNUSparseFile.0/a.img")
    t.size = n * blk
    t.pax_headers = {
        "GNU.sparse.map": ",".join(f"{2 * i * blk},{blk}" for i in range(n)),
        "GNU.sparse.name": "a.img",
        "GNU.sparse.realsize": str(2 * n * blk),
    }
    bio = io.BytesIO()
    with tarfile.open(fileobj=bio, mode="w", format=tarfile.PAX_FORMAT) as tf:
        tf.addfile(t, io.BytesIO(b"D" * t.size))
    return bio.getvalue()

def read_time(n):
    with tarfile.open(fileobj=io.BytesIO(build(n))) as tf:
        f = tf.extractfile(tf.next())
        t0 = time.perf_counter()
        f.read()
        return time.perf_counter() - t0

for n in (1000, 2000, 4000):
    print(f"{n} segments: {read_time(n):.2f}s")

-        buf = b""
+        chunks = []
         while size > 0:
             ...
-                buf += b
+                chunks.append(b)
             else:
-                buf += NUL * length
+                chunks.append(NUL * length)
             size -= length
             self.position += length
-        return buf
+        return b"".join(chunks)