1. Disable your other airport expresses and the 2nd airport extreme
2. Place your iPad as close to the airport extreme as is reasonably possible (the one connected to the cable modem... just to try to get the max speed – it would be interesting to see what the speed dropoff with distance is in a second, separate test)
3. Try the different options again (especially connecting on 5ghz using a different SSID with the airport extreme's 5ghz network set to n-only)
It's not just the wifi interference (which shouldn't be a problem, but could you also post how many other wifi SSIDs are visible from your iPad?) – but multiple airport expresses mixed with an airport extreme falls outside the realm of what 99% of users do (I think it could be a bug when the airports interact).
As other posters have said, this kind of performance drop is abnormal.