You might be staying in an area that's close to the edge of the coverage area for the adsl or not have the full support from your exchange to be on your current line speed? The only way to know is to phone Telkom at 10213 or phone your ISP for assistance.
Use the My BB speed test link at the top of the My BB forum, and post results here. Then perhaps folk may be able to give you input. Here's the link to the My BB speedtest
Use the My BB speed test link at the top of the My BB forum, and post results here. Then perhaps folk may be able to give you input. Here's the link to the My BB speedtest