Ok, a quick update. Been doing pings all over the place and noticed something.

@Kevin_Z, this might give you guys some insight.

1. Devices connected over the Wifi network can ping internet addresses just fine and the response time is consistently good. Ping to 8.8.8.8 (dns.google) has 16ms on average over 40 packages sent, no packages lost. *edit 1 here*

2. Pinging my cabled PC from any of the wireless devices turns out to be fine too. Average is 3ms on extended ping, spikes here and there.

3. Pinging a wireless device from my cabled PC shows the behavior reported here. Response times are high, inconsistent, but no package was ever lost over 30 minutes.

4. Pinging a wireless device from another wireless device shows the same behavior, but a bit aggravated = higher response times.

5. Pinging the router backbone, from either the cabled PC or wireless devices, also provides fast and consistent response times, 2ms on average, and no package losses. *edit 2 here*

At this point I'm 100% sure there is no issue with the carrier, between the ISP modem and the router, the router and external/internet addresses, or cabled devices and the internet. The issue is exclusively between Wifi devices connected over the same router/subnet trying to reach each other, and cabled devices trying to reach wireless ones. Given the fact that the response is good when the source is wireless and destination is cabled, but not true the other way around, I believe the issue is specifically on the handling of the responses from the destination wireless device by the router over the wifi network, either 2.4ghz or 5ghz, on any settings.

I assume the root cause is either a hardware failure on the router, there is an implicit QoS-ish service or protocol harming the wireless network, or it is indeed WMM which I still haven't figured how to turn off without kicking everybody out of the network.