Thank you Garret for jumping in.
Actually, from the moment I posted the message, I also did some search to see if there are other forums where that kind of delay was mentionned.
I found some threads talking about like 10 minutes for a report to just open, and in the numerous suggestions that didn't really made sense to me, one reply though, mentioned the "Verify on First print" flag available in the options.
And, for me at least, this was indeed the source of the problem, as soon as I unchecked it, things went back to normal with that particular RPT.
I can only guess, that as the report was about to be opened, and before any code sets the single record that is processed every time an invoice is printed, that flag had as a result for the entire table to be scanned, probably not using any index(?) and this is what caused the initial delay. At least this is how I explained the problem to myself :-)