I invite you to come check out the most populated official server in US, when there is around 25+ players on tonight, we have this rule of thumb there, 6 people online everything is good, 10+ people its lagging, 15+ now the stairs are your enemy, 20+ good luck.
But it is real interesting what you found, could you tell us about the provider? is it G-portal aswell? and could you by any chance make some video footage of you running up and down some stairs when you have 20+ people connected? what we have found is it is on official servers, completely tied to players connected, we tried another server with 6 people same region, zero problems what so ever, but join any other server with 19+ was yesterday the second highest population, the server dident matter, it was all about how many people were connected, and we even tried a completely different region, same result low population it ran smooth.