Fast ethernet means 100Mbit/s?
Yes. And if the powerline adapter only implements 100 Mbits/sec then of course this is the bottleneck regardless which link speed your NIC in both clients and server support and use.
But even if the powerline adapter negotiates a 100 Mbits/sec link depending on sitation on the power wires real throughput between both powerline adapters on each end can be much lower (bottleneck moved)