Use one thread by default

[superbonaci]

Using 8 threads causes reduction of performance, when reading from conventional hard drives (mechanical). Reduction of sustained read speed goes from 120MBps to 40MBps, and increased hard drive noise is noticed, the arm swings move like crazy.

[paulhargreaves]

Changing the default behaviour doesn't seem to be particularly user friendly to people who are already using it.

[ashugg]

Use the -j flag to adjust the number of hashing threads. If your system is spending too long in iowait, make the number lower. On an 8 core system I'd try -j4 first, then -j2. If it's a mechanical drive connected by USB 2.0, -j0 would be appropriate.