TLDR: I have a configuration issue for a wireless network and I haven't been able to figure out what the issue is

I have a computer running Ubuntu 14.04. I'm trying to see if I can get it to do some video streaming over wireless rather than over ethernet. Everything was working swimmingly over ethernet, so general config should be ok (where the TV is, there's no good way to get ethernet to it, hence checking wifi). Figured I'd test feasibility using the 2.4GHz and then go to the uncrowded 5GHz if it seems feasible (and buy a bridge or at least a usb wifi).

I've got a dd-wrt router that can do 2.4 and 5 GHz, but I don't (yet) have a receiver that can get 5GHz for that box. I /do/ have a laptop that can test both the 5GHz and 2.4GHz networks.

I have a server at 192.168.1.2 that I'm using as a NAS (also Linux).

The box I care about is named "Smoochy", the server is "Serenity" (192.168.1.2), and the test laptop is "Slimer" (I got on an 's' kick for my computers a while back).

So when Smoochy tries to ping 192.168.1.2, I get average round trip times of about 400ms, with worst often being around 1.6 seconds. However, when Smoochy tries to ping google, average rtt is about 50ms with worst of 100ms. Both Serenity and Smoochy are on the same network and are using the same router as their gateway. There are no dropped or re-sent packets. They all get there ... just slowly.

Ok, so I thought maybe I should test this on my laptop that can connect to both the 2.4 and 5GHz networks. I'm sitting in the same place. The 2.4GHz network has an average rtt of about 2.4ms and the 5GHz of about 0.8ms. So that's not the problem.

I thought then that I'd try to use the same usb wifi that Smoochy is using. Grabbed it, disabled wlan0 on my laptop, plugged in the dongle, and connected to the 2.4GHz network. Average rtt? 2.4ms. Crap.

So the main thing I'm left with is configuration (I think?). Smoochy has to be configured via /etc/network/interfaces because xbmc doesn't read network-config. When I boot into the Ubuntu GUI, I can connect via network-config and get fast ping times. 3.4ms for Serenity and 40ms for google.com. Most of the time. Some times it's 80 average, some times it goes back to 500+ average.

Not sure what'd be causing this.

My interfaces file looks like this:
I'd appreciate any help so I can test if this can possibly work wirelessly. The 5GHz network, I can restrict to just high-speed wide-band traffic and there's no one else anywhere around using any 5GHz signals, so it should be clear.

So smoochy has fast pings to serenity over wired
smoochy has slow pings to serenity over 2.4G wireless
slimer has fast pings to serenity over 2.4G and 5G wireless using its internal wifi adapter
slimer has fast pings to serenity over 2.4G wireless using smoochy's wifi adapter

I assume that you're running these tests in the same spot (same wireless strength)?

What distribution and version are smoochy and slimer using? It could be a simple matter of wireless adapter drivers are older/buggier on smoochy than they are on slimer. Have you tried running a liveCD on smoochy to see if a different distro has different results (preferably the same as you're running on slimer)?

Correct summation, though I'd add that Smoochy can also ping /Google/ (which goes through the same router, but then to an external network obviously) "fast" at an average of 50ms, just like Slimer. For some reason, simply pinging on the /same/ network is slow.

Both Smoochy and Slimer are running 14.04 Ubuntu updated on the same day and yes, I physically sat over near Smoochy to test Slimer's wifi.

What os is serenity running for these tests?

Same. Sorry, I should have said that. I upgraded them all to try and eliminate any variables there. Everything being tested there is Ubuntu 14.04 updated as of about Friday evening.