Why does a visa take so long to process?

p>I’m referring here to visas in general – UK, US, Australia etc, the process seems equally lengthy wherever an application is lodged.

Assume that "quick" visas typically take 5 months… what actually takes so long? The usual answer is "short-staffed", but this is just nonsense if you think about it – if understaffing was the reason, the backlog would be ever-increasing each month (assuming a roughly equal number of applications each month) and as a business model this would not be sustainable for more than a short period.

It also cannot be the "processing time" – either 10 documents can be verified which would take a few days to be extremely generous or, as I suspect, checking procedures are started (and, since the documents are often independent, e.g. identity, criminal record check, education certificate check, medical check etc. they can presumably be kicked off in parallel) and the case officer has to wait for the results in days/weeks, whereupon they stamp and issue/deny the visa (nowhere near 5 months). In the meantime, they get on with starting other people’s visa checks since there is clearly a backlog of 5 months’ worth to get on with. I can’t see how it’s not sitting in someone’s in-tray for a good 90% of the 5 months. And what about the 3 year Australian Skilled Migration visas?! So, can anyone shed some light? Businesses who operate like this in the private sector go bust within a matter of months!