Why is my connection slow (throughput)
A few important factors to bear in mind when running a speed test. If anything else on your network is uploading or downloading data at the same time, then the speedtest results will only show your share of the bandwidth (what’s leftover), and not your overall bandwidth. For example if a server is sending/receiving email, or other users on your network are either sending/receiving email, or browsing, or if their PC’s are checking for software updates, or downloading updates, the any speedtest run at the same time will be artificially low.
One common issue with ADSL is if the upstream bandwidth is saturated (heavily used), then the downstream will slow down to a crawl because the ACK packets cant be sent. The way to resolve this is to introduce traffic management in the router (limit max upstream bandwidth usage) so that there is always a few hundred Kbps upstream bandwidth left over for ACK packets etc. You may also add QoS rules to the router to Prioritise critical traffic such as VPN and VoIP over less important traffic such as email and web.
Another factor to take into account is how busy your telephone exchange is at certain times of the day, because ADSL is a contended service by design ( http://en.wikipedia.org/wiki/ADSL_Max#Congestion_and_performance_guidelines ) so throughput speeds will vary as a result. Our “Priorisised” ADSL products prioritise data over standard ADSL data through BT’s systems, so throughput speeds are higher with these products when contention is an issue.