I haven't streaming any movies yet over 5Ghz. I currently don't have the 5500 connected. I hope to soon though as I have testing to do.
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What modes are you using on the 5ghz? Mixed or single mode?
Are you streaming to a device from another device on the LAN side or you streaming from the WAN?
Can you try setting DNS Relay OFF and manually input your ISPs default DNS IP addresses under Setup/Internet and see if this does anything?
Also streaming more than one media stream might impact some performance. Also the resolution or data that your streaming can impact as well. Streaming 1080p with 2000x2000 resolution could be hogging bandwidth...
How far away is the bridge from the 5500?
I'll put my 5500 back on line when I get a chance and see what I find with my DIR-868L in bridge mode.