AFAIK the bandwidth cap is a 'total bandwidth' cap and doesn't allocate bandwidth on a per-user basis. I don't know if it's really possible to do so without a lot of overhead.
So the cap could be 1GB/hour, and all connections are treated equally under that restriction. Therefore the 'speed' of the forums is inversely related to the # of people using it at any given time. And even when there's ~500 people logged on, there's still some variation (few people sit there and *constantly* load threads) and due to random confluences it's possible that you personally could experience a sudden burst in speed.