Theorotically it shouldnt. But practically, Yes.
Every packet sent or received is accompanied by an acknowledgement by the receiver. Now if you are downloading something, ack have to be sent for these as well. At the same time if you are uploading heavily, its most likely that these "ack"s will be choked at your computers output terminal and there maybe a delay in these being sent. This will inturn affect the download. All this wont happen if there is some sort of priority based transmission. There is a software called "Cfos Speed" (www.cfos.de) which addresses these issues. But no promises from my side as to how well this software works.
i have used cfos speed, hoping to increase my internet speed. But due to BSNLs variable speed 256-2000kbps the software only increase the browsing speed. But sadly i dosnt improve the download speed(especially from -- link removed --) . As for now there it is not much helpful. Once BSNL supplies a constant 2Mbps or something then this software is worth using.
cfos speed does increase torrent download speed. use IDM to download from -- link removed -- with servers in following desending orderCogent|Cogent #2|Level(3)|Level(3) #2|Level(3) #3|TeliaSonera #2|TeliaSonera|GlobalCrossing etc...