It would still be faster with a two layer approach. Check 100 000 entries (4 comparisons each) - > filter out 1% to do expensive calculations on. Vs Check 25

It would still be faster with a two layer approach. Check 100 000 entries (4 comparisons each) - > filter out 1% to do expensive calculations on. Vs Check 25 cities (4 comparisons each) - > filter out 1/25 Do another easy run on the filtered 4000 and get the same areas to do expensive calculations on as above. That's 25+4000 easy calculations against 100 000. But yeah, I'm not sure why they seem to go the hard route from the beginning.