@grim_reaper 

The usual culprit is that the airwaves are already jammed with lots of other uses of the 2.4G band.  One way to verify this is to compare 5G speeds to 2G, if the 5G is consistently a lot more, then you probably have congestion in the 2.4G band...which typically you can do very little about.  If they are similar in performance, then the issue is upstream...bad cable/bad connector....packet loss, port set to 10M or 100M instead of 1G that sort of thing.

Lastly, and I tell this to everyone that doesn't live on their own private island, had code your 2.4G channels to 20MHz, this maximizes your chances of drowning out competing APs, while at the same time limiting the number of foreign APs that can interfere with you.  I also recommend you try to stick to channels 1, 6 and 11 (being aware of which channels have the strongest interference at each AP...and picking the one with the least amount of competition).

  @grim_reaper 

Ok, so to summarize:

-unplugging the AP and plugging the cable into your laptop results in full speed

-plugging a different AP into the same cable works as expected

-no difference between the supplied 24V passive injector vs POE switch (even if it is only 100M capable)

When on a wirelessly connected laptop, a long term (30min+) continuous ping shows no lost packets and consistent delay? 

(Windows cmd would be ping -t IP.OF.YOUR.ROUTER ). After 30min hit Ctrl-C and then review the summary stats.