How does it work sometimes that speed tests show decent download speeds but websites and videos are still loading slowly?

Various reasons

1. Routing between sites varies
2. Some speedtests report a burst speed only
3. Speedtests often use multiple threads to give the best possible speeds
4. Website itself may be busy
5. Websites are lots of very small elements, so latency plays a critical part
6. DNS lookups may be slow

However good speedtests allow you to check whether you are getting from your ISP more or less what you should be. While all those aspects that Mr Saffron mentions are beyond an ISP's control.

Right, apart from DNS (some ISPs maintain their own caching DNS servers to speed things up for the EU)

P.S. I find it misleading that ISPs always quote the 'best connection' figure, say you signed up for a 12Mbps service, that 12Mbps is achievable only during the day and late night/early morning.

IMO they should be quoting the average speed you can get in the evenings, as this is when most working people use their broadband, but thats another debate entirely

Edited by matthewcl375 (Thu 14-Mar-13 23:07:54)

Providers are meant to give a personalised speed estimate at signup if signed up to the Ofcom code of practice and in advertising should show the speed that 10% of users can achieve

Yes, BT gave me an estimate based on their database which hasn't failed me so far, it predicts 6.5-9Mbps sync atm which is spot on (6.88Mbps) and is predicting 16-19.5 for ADSL2+, but I've heard many say these figures are often an underestimate

In reply to a post by matthewcl375:

Right, apart from DNS

Yes, I thought that as I posted but they are easily overcome w/out troubling the ISP.

In reply to a post by matthewcl375:

P.S. I find it misleading that ISPs always quote the 'best connection' figure, say you signed up for a 12Mbps service, that 12Mbps is achievable only during the day and late night/early morning.

No, generally your sync speed does not change day-in day-out. You are mixing it up with throughput. Haven't we been here before?

BUT all advertising and your personalised quote at sign-up is in terms of sync speed or "access line speed" as Ofcom calls it.

@MrSaffron:

In reference to stakeholders.ofcom.org.uk/telecoms/codes-of-practice/broadband-spee...:

the Code encourages the ISPs to provide consumers with information on their access line speed *

* access line speed - This refers to the maximum speed of the data connection between the broadband modem and the local exchange or cable head end.

Now it may be that it is this you find misleading but, for the reasons we've just discussed, reliable throughput figures are hard to predict by the very nature of the Net, and Ofcom & ASA have accepted this.
EDIT:

In reference to stakeholders.ofcom.org.uk/telecoms/codes-of-practice/broadband-spee...:

Ofcom is also working hard to find a means by which this information can be supplemented by additional information, including on the average throughput speeds obtained through different ISPs, since these speeds are likely to vary for a number of reasons. If appropriate, the Code will be revised as a result of such further work.

Edited by XRaySpeX (Fri 15-Mar-13 01:40:10)

I believe those estimates you are quoting you obtained from the public BTw Checker.

But, as a matter of interest, were you given a personalised quote at sign-up to BT?

I was, they gave my dad a quote by email and on their website of 17.5Mbps

Just got home to see this on the router log:

Mar 15 08:23:00 xDSL linestate up (ITU-T G.992.1; downstream: 7616 kbit/s, upstream: 448 kbit/s; output Power Down: 13.8 dBm, Up: 11.9 dBm; line Attenuation Down: 6.0 dB, Up: 6.0 dB; snr Margin Down: 6.3 dB, Up: 23.0 dB)

Looks like I've hit my max sync with interleaving enabled However I am now seeing more FEC 'errors'

Edited by matthewcl375 (Fri 15-Mar-13 19:23:46)