Powerline is the spawn of Satan in networking terms, that you get horrible performance from them even in the same socket is a powerline issue, nothing to do with your router. Test with nothing plugged in/connected on the gigabit ports directly connected and you’ll get 930-940Mbit (1024Mbit less overheads), the 2.5 port will show roughly 2.3Gb (2.5Gb less overheads) as you found. In terms of what you can change, there are a few wireless settings on the EX820v, have a look: Getting the Best Out of Your EX820v Router with a Few Tweaks
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