subprocess.py (Python 2.5, current SVN, probably all versions) contains this O(N**2) code:

  bytes_written = os.write(self.stdin.fileno(), input[:512])
  input = input[bytes_written:]

For large but reasonable "input" the second line is rate limiting. Luckily, it is very easy to remove this bottleneck. I'll upload a simple patch. Below is a small script that demonstrates the huge speed difference. The output on my machine is:

creating input
0.888417959213
slow slicing input
61.1553330421
creating input
0.863168954849
fast slicing input
0.0163860321045
done

The numbers are times in seconds.

This is the source:

import time
import sys
size = 1000000
t0 = time.time()
print "creating input"
input = "\n".join([str(i) for i in xrange(size)])
print time.time()-t0
t0 = time.time()
print "slow slicing input"
n_out_slow = 0
while True:
  out = input[:512]
  n_out_slow += 1
  input = input[512:]
  if not input:
    break
print time.time()-t0
t0 = time.time()
print "creating input"
input = "\n".join([str(i) for i in xrange(size)])
print time.time()-t0
t0 = time.time()
print "fast slicing input"
n_out_fast = 0
input_done = 0
while True:
  out = input[input_done:input_done+512]
  n_out_fast += 1
  input_done += 512
  if input_done >= len(input):
    break
print time.time()-t0
assert n_out_fast == n_out_slow
print "done"