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This is something that is being omplemented right now, but it also has to do wth the way the block is updated. this is done using the garbagecollector, that has a 10% chance of updating the block info every time the block is displayed. With few visitors, this means the block is updates less often, and thus slower. With many visitors....
Do a search for the who is online block in the forums, aor for garbage collector, and you can find descriptions on what file to adjust to make this go faster.
Herko