PlusNets traffic management systems are what is causing the slowdowns, together with the increasing amount of encrypted P2P which has to share the bandwidth with HTTP etc thus leaving less bandwidth for the lower classed P2P/FTP/Binary usenet connections.
PN are testing some new Ellocoya software which may help distribute the P2P traffic better which should improve the overall speed.
The bandwidth usage graphs
here show the impact P2P is having on the network. The orange band is detected P2P and the encrypted P2P makes up the majority of the pink (other) grouping. Together they ad dup to over half the bandwidth usage. As a resulr PN have to limit that type of traffic so normal web/email activities are not affected. If they did not, all your traffic, including web access, would be a a crawl.
P2P does slow down during the day but does increase to an acceptable level overnight. That is how the traffic management is designed to work.