Powerline data loss?

Today I dealt with my internet again because I often have a very low download rate. I'm among other things I noticed that I have a higher download rate (78 mb / s, with a tariff of 100 mb / s) with my Wi-Fi on (tested with laptop) than with my PC connected via Poweline (28 mb / s, for downloads or Installations only 4).

I made sure that I avoid any mistakes, like Poweline over multiple sockets.

Where else can the problem be?

If the problem does not exist with a direct connection (i.e. Without a powerline), then it will be with the power line.