Actually it could speed up YOUR downloads, while it slows OUR downloads plus a net lost in overhead bandwidth that do not serve either downloads or uploads of either you or us because multi-networking has to substain a lot of connections. That's why Shareaza is such a problem, some people are selfish (Multi-network users) and the others (Gnutella users) pay because of that. That's the beautiful spirit behind the stupid idea of multi-network support
Shareaza is really meant to be used only with G2, some devs are just running after hype and popularity.
Shareaza + G2 = ok
Shareaza + G2 + another protocol =