I do have the same problems her to
i have 8 users on a RB532 and this is on one sector of 4 , i have tryed every chanel but no luck i have switch the antenna and coax and the radio card.... the only thing left is the RB532 card... the speed on that sector is max 512 kb to 1 mbits...
the oteher 3 ones have 12-25 mbits...
At least i'm not the only one
The only thing i noticed that's strange is that even if bandwith usage is higher on download then on upload (far higher, up to 6:1 ratio) the packet rate is pretty much the same (and high, around 1000 pps at 2 mbps which is REALLY high in my experience).
as it happens to you some other sector are much more loaded (up to 38 clients) and use much more bandwith, with no problems.
I suspect the problem is p2p related, as we do not limit p2p usage (nor we want to).
I'm trying a thing to fix it, so far it seems to be working.. cross you fngers, will report..