As the population of this server increases I have noticed a performance decrease with placing blocks, opening inventories, etc. While some of this is expected, it seems pretty drastic in the past couple of days.
This thread mentions Modular Power Suits as a possible culprit, and I've read other threads mentioning it as well.
Basically, MPS sends info directly from one players client to all other players clients, so every added player dramatically increases bandwidth used.
Something to consider.