If you see a contention ratio of, say, 50:1, this means that 50 subscribers share that bandwidth. The more subscribers sharing the bandwidth then the slower your connection will be. 50:1 is common for a residential package. However you may prefer to pay more for a service with a lower contention ratio.
Business packages often have lower contention ratios, eg of 20:1 or even 10:1, because it is essential to them that they have a continually fast service.
The contention ratio certainly isn't the only factor determining download speed - this could be down to the distance from your exchange, or to another factor - but it could be part of the cause of slow speeds, or the main cause, particularly if your sharing subscribers are heavy users......