because you ruled that out already, it's a router issue or a bad reception (possibly caused by interference with other radiation sources).
I'd try (a) changing the channel on the router, (b) walking around with a different laptop for a better diagnosis , (c) try another router, and (d) try this PC/laptop somewhere else. The last experiment shouldn't make any difference, of course, because you ruled out it's an issue of this PC. So it's just a double (or maybe even triple) check.
Our internet connection over the Wi-fi (router VWR-VC) is extremely slow. We have ruled out all virus/malware and PC issues. In fact, when we disconnect the wi-fi, the internet connection on our PC runs perfectly.
The wi-fi connection is secured with a id/password.