So...I have the exact same issue. I can vouch for what OP is seeing. I even made an account just to post on this thread.
It seems to have problems with TCP. Youtube videos brings the throughput to a crawl. Using packet sniffers I can see the SYN/ACK keeps getting reset and there are duplicate frames. It actually kinda looks like the DAP-1525 is doing a DoS attack on my computer.
I have tried many setting configurations. I even swapped my DAP-1525's to see if there was a hardware problem. Each DAP-1525 displays the same behavior. The DAP-1525 interfaced with the router (DIR-655) routes fine but the other side of the bridge is getting TCP stormed with duplicates.
Your device has an IPv6 bug... a very silly one.