I think it is also due to perhaps a bad DSL connection, where both download and upload bandwith are low.
Therfore I suggest the following improvement:
1) low and high quality (custom) contents, especially for skins and models
a. models and skins should have a low-res and a high-res version
b. Maniaplanet would detect the available bandwith and automatically select the low or high detail version
for both downdload of foreign skins and upload of the own skin,
unless the user specifies the one or the other as constant configuration
2) servers could host models & skins and perhaps also mods based on player session duration
a. a player connects, his skin is uploaded into the server cache
b. other player connect, and the skins of all already connected players are downloaded from the game server
3) skins and models should always use ManiaLink
3.1 the Maniaplanet player page should have a storage
for a skin and perhaps for a custom model per game (TM/SM/QM)
which the player would configure either by direct upload
or
3.2 by providing a separate ManiaLink so the maniaplanet server gets the files
from the player's real location
Of course this would not reduce that much the downloads to the player's machine,
but it would considerably reduce the P2P uploads, especially with low bandwith,
and for those who still absolutely want P2P enabled
