I started experiencing "slow speeds" after moving on to 21CN at end of last year (not sure if it was related or just coincidence).
After migration I noticed that web browsing and downloads seemed to be intermittently slow. At first I just thought it was the sites I was using, but eventually investigated and ran some speed tests using TBB and Speedtest.net. These were very variable: sometimes getting throughput of around 9Mbps but often between 1.5 to 2.5Mbps. Line sync speed was solid at around 15Mbps with profile of 13000kbps. On ADSL Max I had been getting a rock solid 6Mbps.
After a while I noticed a pattern to the speed variations: I got 9Mbps throughput reliably at evenings and weekends, but it dropped to a quarter of that or less during office hours, Mon to Fri. Both TBB and Speedtest.net gave pretty identical results, and the results were consistent with results I achieved when downloading files from other sites. The ADSL line is just used by a single Mac computer, so no other traffic affecting the results. All very strange!
I then tried the BT speed tester. That gave consistent results of around 9Mbps at all times. However my understanding is that the BT speed tester is effectively connected in at the exchange level, so it's really just measuring throughput of the ADSL circuit, not the end-to-end throughput including the BT WBC and Zen core networks.
Then spoke to Zen tech support last week and explained these points. They basically said the BT line test result was OK, so my line was good. Neither BT or Zen would place any credibility on speed tests from TBB or Speedtest. The problem must be the speed test servers themselves being under load during office hours (unlikely IMO). So therefore I didn't have a problem and they weren't prepared to investigate.
I was pretty unhappy with this response. As a result of that and the fact that I'm now bumping against my download limit each month with Zen, I decided to ask for my MAC code. Pity, as I have been a very happy customer for 6 years until now.
Line was activated on Be this afternoon. Sync speed is increased to 19,598kbps and Speedtest shows a throughput (during office hours!) of just over 16Mbps. That's a bit better than the 1.5Mbps I was getting on Zen at the same time of day yesterday