Extremely slow 'processing file' times for Apps in iTunes 10.6
I too have seen this on multiple macs with multiple OS / iTunes versions in the last few days. No amount of client changes seem to resolve it (the downloads themselves don't need to be cleared, clearing them doesn't help, and the situation does resolve itself in time with no user intervention. Usually the wait is 5 to 15 minutes to clear in my experience.) It seems to happen at the same times when my iOS devices struggle to re-download past purchases from the store.



My guess is you are seeing a delay in the code signing checks which are being queued on the server side to wait for their turn. It's clear that the download is very rapid and the app is done downloading the app. 



My guess is Apple now serves identical apps (unsigned to your account) to iTunes on your computer. Then your computer requests that that app get signed to run under your Apple ID and those requests are queued and that is why nothing is happening on iTunes while "processing" is taking place.

I have seen this come and go and presume it's a capacity issue on the Apple server side and not anything new or changed on our iTunes computers.

Anecdotally, there have been all sorts of errors and slowness in reaching the store lately. I would love to see the server logs for loads and error rates to know if this suspicion is correct. If my guess is correct, this will only happen when the download volume exceeds the servers that are running to do the signing. If so, it will be easy for Apple to spin up more signers as needed to meet demand in the future (as well as scale back to save costs when demand is lower than say iPad launch weekend).