ами да ти кажа не виждам голямо разминаване между нашите мнения: ако софтуера е multi-threaded, тогава всичко опира до ОС-а. Ако не е - негов си проблем, ако това е голяма програма според мен е много лош дизайн.
И тъй като големите програми обикновенно са multi-threaded, почти всичко зависи от ОС-а
ето ти извадка от една статия по въпроса

"Operating Systems like DOS, Windows 3.x, and Windows 95/98/Me are not “aware” of a second CPU at the hardware application layer and will never recognize it. If you use a 32-bit pre-emptive multi-tasking OS like Windows NT or 2000 and have a second processor in your computer you will only see about a 50% increase in speed over a single CPU system. The caveat is this increase will only be with software that is written for a second processor specifically as a multi-threaded application. High-end graphic suites like Photoshop are frequently multi-threaded due to the rendering that they must perform although most other applications are unfortunately not. To add insult to injury NT 4.0 had a flaw that would bounce a single thread between CPUs creating great inefficiencies. This flaw is said to exist on Windows 2000 as well.

Current versions of Linux and Unix do not have the problems of NT and 2000 however as most all applications are multi-threaded and benefit from multiple processors sharing application load. Unix can take advantage of just about as many processors as you can throw at it because the kernel uses a heavy threading model and can assign these instruction sets to multiple CPUs. This architecture is much more efficient than Windows and the speed increase proves it. The same is true with Mac OS X with its BSD core although Mac OS 9 requires dual-processor aware applications in much the same way as NT and 2000 do."