The thickness of the copper cable determines the signal transmission loss per km, thus impacting on the maximum attainable speed for a given distance. The maximum attainable speed using DSL technologies also decreases as the length of the copper increases. Many Telkom DSL customers wondered why they could only get a maximum speed of a few Mbps when their exchange is 10Mbps or even 40Mbps enabled. The maximum speed is a product of three main factors: the quality/thickness of the copper, the length of the copper (distance from the exchange/MSAN), and the technology used.
Any DSL line fault can cause a drop in speed. This is particularly common in cases where DSL line is disconnecting regularly. As mentioned above, it can take up to 3 days of stability before DSL line speeds return to normal. DSL line is supplied over your telephone line, so it needs the line to be working properly. Subsequently, it's common for telephone faults to cause problems with DSL line service. You may see slow speeds or you might not be able to connect at all. In nearly all cases, once the telephone line is fixed the DSL line will be too. Connection problems tend to clear up immediately, with slow speeds it'll be a few days before your speed gets back to normal.