They are so fast, that I change strategies on which line to choose. Usually less stuff to be checked out in front of you is good, but at aldi I go for least people, no matter how much stuff they loaded. This might be even more significant in germany, where many pay cash and you get every cent back in change.
Your strategy, especially at Aldi, is the way to go. I read an article about a decade ago that was challenging grocery store "express lane" speed. Since groceries have been using the same kinds of scanning mechanisms at every chain for a very long time, the barcodes are usually optimally placed to allow for the "scan/bag" to happen quickly[1]. The article identified that the bottleneck was more often the transaction times and not the time to ring up an order meaning that an express lane with three people in it would take longer than a lane with one person and a cart full.
Granted, this was a decade ago and many things have changed since then, including the "super store" which includes a lot of products not optimised for scanning in the first try.
[1] Cashier experience plays a roll here, as anyone who's shopped at Walmart can attest. At Aldi, though, solves this by having the barcode occupy almost the entire length of one side of the package, making it harder to miss the scanner than hit it.