The internal wirings may play a significant role in internet speeds. If there are unused phone lines distributed through different rooms , try to eliminate them and use a decent splitter wherever you have a phone connected and where your modem is connected.
I had such an experience about two years ago. My well working internet connection started not to work anymore one day and the line dropped quite often. Then I had the wires going to upstairs rooms disconnected and had a healthy connection again.
I don't think the builders allways use top quality wires thus their performance may downgrade sometime through dampness , corrosion etc.