Default: auto (frame based threads: 1.5 * logical processors, rounded down; slice based threads: 1 * logical processors)
Enables parallel encoding by using more than 1 thread to increase speed on multi-core systems. The quality loss from multiple threads is mostly negligible unless using very high numbers of threads (say, above 16). The speed gain should be slightly less than linear until you start using more than 1 thread per 40px of vertical video, at which point the gain from additional threads sharply decreases.
x264 currently has an internal limit on the number of threads set at 128, realistically you should never set it this high.
Recommendation: Default
See also: thread-input, sliced-threads [edit] sliced-threads
Default: off
Enables slice-based threading. This threading method produces worse results than the default method both compression and efficiency-wise, but adds no encoding latency.
The maximum number of sliced threads is MIN( (height+15)/16 / 4, 128 )
Recommendation: Default (off), unless you are doing some sort of realtime streaming or low latency is important.
NOTE: the "thread count" listed below does not count the lookahead thread, only encoding threads. This is why for "veryfast", the speedup for 2 and 3 threads exceeds the logical limit.