It looks like someone else on Reddit may have reported a similar issue years ago...

https://www.reddit.com/r/HomeNetworking/comments/s77lxk/powerline_connection_causes_entire_network_to_lag/

I have a cable modem (+ router) in my basement which I've been extending to my room (ground floor) through a PowerLine configuration for more than a year w/o problems (hard to believe, i know). Recently however, I've been experiencing very freqent and consistently timed ping spikes (see picture) or complete dropouts on both the PC connected to the modem directly and the one connected to the PowerLine.

I've ruled out pretty much every electrical device by systematically disconnecting breakers, and tested the quality of transimission through the wiring from the basement to the ground floor, which seems to be completely fine (4-5 ms without any changes). I noticed however, that ceasing ANY kind of PowerLine connectivity in my house instantly solves all the ping spiking (the PC in the basement was my testing ground for this), regardless of which socket the PowerLine Unit connected to the Modem is plugged into. I've confirmed this with any combination of my 3 Netgear AV500s and a TP-Link AV600.

I've read about PowerLine disrupting DSL/VDSL Modems, but I'm using a cable modem over Coax, and have put as much electrical distance as physically possible between the two systems with zero changes. At this point I am simply at a loss for solutions, as nothing has changed between the year i have been using the system successfully for and now. I hope someone can shed some light on the situation.

I found another reference ... 
 

Ulrar 1y ago

I've had that here, I had a unifi switch plugged into a powerline adapter (so powerline between the UDMPro and the switch) and it caused these spikes for the whole network, even devices not connected over powerline. It seems unifi gets real weird if the connectivity to one of the managed devices is bad, or at least it did for me. Proper ethernet everywhere solved that
 

https://www.reddit.com/r/HomeNetworking/comments/zbx4mf/comment/iyurlvj/

Looks like another case ...

https://www.reddit.com/r/HomeNetworking/comments/8loffh/tplink_powerline_adapter_causing_latency_on/

TP-Link powerline adapter causing latency on primary network? (Wired devices to Fios Router, that do not interface with the powerline adapter's hosted wireless network, are affected)

To preface, I am not a networking guy but work in IT. I'm going to bullet point the situation so it's easier to digest. Here is a little diagram I created in paint to give a visual aid.

1. I pay for Fios Gigabit Internet

2. I use the Fios Quantum Gateway router

3. I use a TP Link AV600 powerline adapter so the Firestick in my living room has access to a network. This is a separate SSID than the "primary network" as I put it in the title.

4. My desktop is directly wired to the Fios Quantum Gateway router

5. Powerline adapter is plugged in directly to the router on one end, and the other end that broadcasts the wireless signal for the TV to connect to is plugged into a different outlet on the other side of the house

6. When the powerline adapter is in use, I get extreme latency while trying to game on the wired PC connected to the Fios router

7. The powerline adapter is NOT in between the PC and the Fios router

8. I came to the conclusion that the latency on my PC resulted from powerline adapter by unplugging the powerline adapter and pinging my router (ping -t 192.168.1.1); I played a game for about 15-20 minutes and didn't have one spike over 2ms, 99% were <1ms. I then plugged the powerline adapter back in and started streaming a random Netflix show on the firestick and the latency from my desktop to the router almost immediately jumped to 300-600ms. Streaming a Netflix show from a device on the primary network does not affect latency, or at least extremely marginally.

The only thing I can think of is that the router is not routing the packets correctly which is causing a delay during that process? Would a network switch help with this, or would that not be any better than the router's switch? Would creating a VLAN for the powerline adapter traffic help? I'm probably completely off here but was the first thing I could think of. I cannot figure out why my wired connection between PC and Fios router would be so heavily affected by traffic on the powerline adapter's connection.