I too am experiencing this problem, and if I had to guess, many people are.
The way that alt.binz handles the primary and backup is really painful, as if there are a number of blocks missing from the primary, I notice that eventually, most of my connections are trying the backup, and often what happens is the backup server blocks me out and I have to wait until it times out those connections.
Worst of all, if there is a propagation issue and the blocks are missing from the primary and backup, each missing block is retried multiple times (I have mine set to 3 retries) and also contributes to the slow down.
I don't know what the solution is here, but I thought I should add my comments in case the more people who have issues, might give it higher priority? Well, I'm hoping it does :-)
Thanks