Your BB speed maybe 20 meg however your upload is much greater reduced.To check the diffrence on upload and download speeds try this:
http://www.speedtest.net/
Rapidshare is generaly slow and this also depends on what time of day it is,how large a file you are downloading/uploading.There are many factors that reduces the speed.How busy there server is,depends on what time of day you choose to download as diffrent times of the day your speed can be reduced.
Post results of the link I just posted above please.
This could also be to do with your computer specifications and are your drivers upto date?