What I do find really annoying is that email that say priority isnt guaranteed, why offer a service if you cant provide it. I feel like that's just blindly robbing us. Saying oh by the way I know you paid for this but I cant guarantee it. I feel like I will be getting an NSF email annoyed and here comes the weekend. 
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What they mean by that is that all priority does is get your application put to the front of the line, so it is picked up earlier than non-priority. That's where the priority service ends... so all you are doing is paying to jump some of the queue.
If your application is then classed as non-straightforward (i.e. missing documents, or criminal convictions, or previous refused visas, or perhaps there are children involved), then all bets are off, and it takes as long as it takes to be decided... which is why they don't recommend using priority if you have had criminal convictions or refused visas in the past, so that you don't go into it with expectations of a fast turnaround. However, we do recommend using priority anyway, because even with a non-straightforward application, using priority is likely to be faster than not using it.
