Ethernet connection slightly handicapped compared to Wi-Fi

I'm using an Arris TG862 router and noticed that computers connected to the internet via an ethernet cable suffer some problems when browsing that don't happen when connected through the Wi-Fi.

The cases that I was able to reproduce were:

• Upload on Imgur.com or Imageshack.com almost always fails
• Upload on Gfycat and other webm hosting sites almost always fails too
• Windows update always fails with error 0x8024401C

It's worth noting that those problems go away when the connection is made through a wireless adapter (even on the same computer).

What could be the cause of this behavior?

Here are some screenshots from the router's configuration page (I can provide more if it's needed to reach a solution):

http://imgur.com/a/IOHTn