I got 99 problems but my WiFi ain't one

I assume you have a bit of cat 5 between the router and Powerline adapter? Has that failed to negotiate the same duplex setting at each end? That would cause poor throughput.

Or do you mean the reported Powerline speed in some management utility is lower? In which case turn them on and off?

Or maybe the router is kicking out a lot of noise which is interfering in which case move them physical apart and/or add a switch between them or go hardcore with fibre.