With line buffering, I see the issue.

 * 6 s without thread
 * 115 s with the spinning thread (varying: 60 s, 98 s)
 * 16 s with the spinning thread and the last "gilinter.patch"


# Modified version of the test case, with bufsize=1

from threading import Thread
import time

def writenums(f, n):
    start = time.time()
    for x in range(n):
        f.write("%d\n" % x)
    end = time.time()
    print(end-start)

def spin():
    while True:
        pass

t1 = Thread(target=spin)
t1.daemon=True
# Uncomment to add a thread
#t1.start()

# With line buffering
writenums(open("./nums", "w", 1), 1000000)