Recently, I've decided to connect my laptop at home to my modem/router through Ethernet cables built into my apartment. As these built-in Ethernet cables are usable via plugs built into the walls (called powerline adapters), I use some additional cabling (STP CAT6 cables) from my laptop/router to the plugs. In addition, I don't know any technical details about these built-in cables (as they're not visible), only the maximal bandwidth (1 GBit => Cat6?) that they're supporting. The built-in cables are Cat. 5e Ethernet cables.

The problem is now that when I set up the connection, I cannot use the full bandwidth that my internet connection is actually offering (100 Mbit/s), only about 85 Mbit/s between 85 - 88 Mbit/s, despite the fact that all cables involved should support the maximal bandwidth. I know from other tests with one of the Ethernet cables in use that I can use almost 99 Mbit/s when I connect my laptop directly to the router/modem using only this one cable. In addition, it is very unlikely that the length of the resulting cable connection from my laptop to my router involving the built-in cables exceeds 100 m.

So, what possible reasons for this loss of bandwidth exist? And how I can fix this issue?

I hope this might help resolve the issue: The built-in cables appear to be Cat. 5e Ethernet cables. So, is there any problem when I connect Cat6 Ethernet cables directly to Cat5e Ethernet cables via the plugs?