Two words stand out in your post: CPU-bound and threads. So I have to ask: are you running this on a 4(or more)-CPU system? If not, you're not going to be able to see any improvement. If you have a good mix of CPU and I/O-bound, you could, as the CPU can be doing stuff while the hard disk is reading stuff in, but I'd expect that the optimal number of threads here might be only 2, in a single-CPU system, not 4. I notice, for example, just with your sample code, that if I run with more threads than I have cores, the speed dramatically drops. Mind you, I don't have any huge log files to use against it, so it's still fast.