What do you mean by dual processing tasks? Are you referring to programs optimized for two processors/cores?
If so, then I would find that a bit hard to believe seeing as how Macs have dual-core processors in them (pretty good ones too). Granted, older programs were written primarily for the single-core PowerPC processors, but current and future programs should definitely be written to take advantage of multiple thread processing.
Do you have any programs in particular you wanted to mention or are you simply sharing some information you "heard" from "somewhere"?
Well, the example you've given would be more dependent on the hard drive and memory speed, rather than the processor. Both these components are on the highest consumer spec available (SATA and DDRII 667Mhz memory) on iMacs and above.
Most programs now support multiple cores very well, otherwise there would be no point in Apple selling 8 core Mac Pros.