Setting a processes priority will have an impact, but only if there are other programs running which are dominating the CPU.
There are a number of programs which can alter other programs processor priority and affinity automatically (including this one
that I wrote, or this other one
that I've heard some good things about but have never tried.)
If you computer meets the recommended specs for WoW and WoW is still running slowly then malware may certainly be the culprit. Run a full system scan with your antivirus (fully updated) and possibly try one of the good online scanners like this one: http://housecall.trendmicro.com/
If the scanners find anything they can't remove or if you feel as though you may be infected anyway, you may post a malware analysis log
in the Malware Logs forum