The issue is that to write to a file, you basically need to serialize the writes. Multithreading will only help you for compute tasks that are CPU-bound. Writing to disk is very very very **very** slow, even a SSD, to give you an idea:
>From [What every programmer should know about >latency](https://gist.github.com/jboner/2841832) Latency Comparison Numbers (~2012) ---------------------------------- L1 cache reference 0.5 ns Branch mispredict 5 ns L2 cache reference 7 ns 14x L1 cache Mutex lock/unlock 25 ns Main memory reference 100 ns 20x L2 cache, 200x L1 cache Compress 1K bytes with Zippy 3,000 ns 3 us Send 1K bytes over 1 Gbps network 10,000 ns 10 us Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD Read 1 MB sequentially from memory 250,000 ns 250 us Round trip within same datacenter 500,000 ns 500 us Read 1 MB sequentially from SSD* 1,000,000 ns 1,000 us 1 ms ~1GB/sec SSD, 4X memory Disk seek 10,000,000 ns 10,000 us 10 ms 20x datacenter roundtrip Read 1 MB sequentially from disk 20,000,000 ns 20,000 us 20 ms 80x memory, 20X SSD Send packet CA->Netherlands->CA 150,000,000 ns 150,000 us 150 ms Notes ----- 1 ns = 10^-9 seconds 1 us = 10^-6 seconds = 1,000 ns 1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns Credit ------ By Jeff Dean: http://research.google.com/people/jeff/ Originally by Peter Norvig: http://norvig.com/21-days.html#answers Contributions ------------- 'Humanized' comparison: https://gist.github.com/hellerbarde/2843375 Visual comparison chart: http://i.imgur.com/k0t1e.png Run On a 3Ghz CPU (i.e. ~3 billions basic instructions per second), a basic operation like add, sub, or, and, mul, shift, xor, if/then/else takes 1s / 3e9 instr = 0.33 ns/instr. Assuming a SSD at 150000 us / 0.33 (ns/instr) = 454545 basic operations in the time required to write 4kB to a SSD. Unless you have **very heavy** processing to do before writing to disk you don't need to parallelize.
