When my latency increased from 7 to 20 earlier this month I raised a ticket with Zen. Here is their response:
Zen servers work to ensure that latency is spread as evenly as possible between our customers. Rather than have one customer with high latency and 10 with tiny latency we want our servers to ensure that no individual has significantly more or less latency than anyone on the server as nobodie's traffic should be prioritized.
Multiple online sources state latencies of less than 50Ms as good. A latency of 20Ms is universally considered excellent or even unrealistic to aim for. Your latency could have increased due to a number of factors, but the most likely one is that usage in your area increased so to ensure everybody had a good browsing experience everyone's latency increased slightly to allow for the greater throughput needs without impacting service.
If internet traffic increases in your area further your latency is likely to increase along side that in order to ensure everyone has the best browsing experience. We cannot save particularly good latencies for individual customers as it would give all our other users a worse experience. If your latency regularly reaches 200-500Ms unloaded then there may be a loading issue on your line that could need investigation and addressing, but with it still being at above excellent rates we are unable to take further action.
David
BT (poor) -> Zen (excellent) -> O2 (started well, went downhill -> IDNet (No complaints - but 100GB cap) -> Zen (gone downhill)