This is commonly stated but not true under all conditions. The full-blown GMail UI has extensive latency-hiding capabilities. The basic HTML UI has no latency-hiding features of any kind. If you are on a high-latency connection but you have some bandwidth available, you will have a much better experience with the full UI. Otherwise you face the full latency for every action.

The Inbox UI is for some reason irredeemable. It is slow under all conditions.