According to history such as Gene Andahl, any gain by improving areas that are already capable of the workload usually has little impact. I can't find the exact article so here's a starter on that area https://en.wikipedia.org/wiki/Amdahl%27s_law
To see if it's the powerline dropping speed (I take it that they are delivering 200) you test with a direct good Ethernet cable.
I have 100 speed into the house and use XAV2101 powerline adapters rated at 200 to get ethernet to my PC which comes through at about 35. Would switching to an adapter rated at 1000 up my speed?
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