Turn around time for PIP decision

I received confirmation in an email that all my information was received in relation to my PIP claim. I was told that I would hear the decision within 6 weeks.It is now moving into 6th week I still have no decision. I note that some applicants have received positive decision in within a week or two of submitting their information.Is there any correlation between the length of time you have to wait and the nature of the decisions? Or is it simply due to the fact that regions differ in their caseloads. The regions with more cases take longer. Just getting a bit worried when I read the speed which some of the positive PIP applicants were processed. In a nut shell,is there any evidence of people who are refused PIPs waiting longer to get their decision?