I am amazed that your customers receive their packages within 5-7 days almost anywhere. (unless they are 'letters')
Priority is only quoted as 6 to 10 working days by USPS for most destinations. They don't even quote an estimate for any 1st class at all.

We've had plenty of Canadian collectors on this bb commenting it can take weeks to receive mail from the US.

The processing within the UK is amongst the fastest anywhere. My comments were based on the fact that I often track incoming packages over 4LB that are not sent priority, and they almost always take 4 weeks plus. When I analyse the tracking I see it being re-routed all around the US before finally departing, whereas a priority (and particularly a fixed rate) will have an almost direct exit route.
Once it lands in the UK, typical customs delay is between 2 hours and 2 days, after which it always arrives here next day. Therefore, it is almost impossible for express or guaranteed to arrive any faster. (in fact if you read the TOC for guaranteed it states that dutiable goods can't be guaranteed anyway so that service seems a waste of money to me - if you are sending just paperwork then Fedex is 40% cheaper).

Even on regular letters, there is a distinct difference depending on whether the sender remembers to attach an air-mail sticker or not.
I acknowledge that they don't physically use sea any more (although there are certain occasions like the Icelandic ashcloud with all flights suspended that they had to), Ive seen nothing to suggest that First Class International is any better than the old economy.

I don't know what is wrong with Italy. It is just about the slowest place for me to ship to also, and my packages to Italy don't even involve customs. Has always been that way. The postmen must walk backwards