Extremely slow download (App-Store & Internet)

I did an EtreCheck and got one major issue:

Top Processes Snapshot by CPU:

    Process (count) CPU (Source - Location)

    Mail 152.57 % (Apple)

    corespotlightd 88.46 % (Apple)

    Other processes 69.45 % (?)

    accountsd 6.72 % (Apple)

    EtreCheck 4.72 % (App Store)

The problem is, that when I download something from the App Store or from the internet it is very slow. (Word took all night)

Also when I connect my Macbook with the wifi, it slows down the internet and on other devices the internet is barely usable.