Need help troubleshooting speed throttling

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New Around Here
Hey guys,

So I have a bit of a speed issue I'm struggling to find the root of. I figured I might be able to find some help to help me troubleshoot here.
I have gigabit internet, but only get about a third of that speed as soon as a couple of devices (but not any device) are connected to the network. My setup is an RT-AC1900P (Merlin 384.18) + CM1000.

- With only one PC wired (let's call it PC1), a speedtest will yield 900+ Mbps, as expected.
- Then I connect a second machine to the network via wifi (PC2), then speedtests results on PC1 immediately drop to about 300 Mbps as soon as PC2 joins the network.
- Bandwidth monitoring shows PC2 is basically idle, no traffic in or out, it is not using any of the bandwidth, yet the router seems to throttle PC1 to roughly 300 Mbps as soon as PC2 connects and I'm not sure why.
- Even after disconnecting PC2 from the network, it takes a little while for PC1 to get back to 900+ speeds, it is not instantaneous. (I'd say maybe 15+ minutes or something), as if the router needed time to close connections or something.
- I tried connecting phones and a chromebook instead of PC2 to replicate with other devices, but that doesn't seem to trigger the speed drop, only PC2 does (I don't have a third PC to test that further tho)
- If I connect PC2 via a wire, no issue, the speed stays at 900+. So it's really connecting a PC, via wifi (5ghz or 2.4ghz is the same), that impacts speed on PC1 for some reason.

So based on that, it seems to be a router limitation, but I'm not sure what it is, still has 50% memory free, so memory availability doesn't seem to be the reason, cpu load is also around 0-3% with both machines connected and idle. QoS is off.
What am I missing?

Appreciate any help getting to the bottom of this, thanks!

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