Asus RT-AC58U router very poor download speed. The fix from the FAQ

The Asus RT-AC58U wireless router though quite powerful, at times causes download speeds to plummet drastically. The broadband connection does work at full speed when directly connected to the computer.

Which means, the router has an issue. Digging further, found the answer to fix the problem in the Asus FAQ at

Use the information to choose another Control Channel.