High at router; slow at PC

I am looking for a solution please to the fact that there is some 17Mb coming into the router but, sometimes only1Mb, or even less, at the PC. It varies a lot: sometimes getting around the 17Mb mark. I can't see the reason for such ongoing inconsistencies.
An iPhone is on the Wi-Fi and that shows around 15Mb. when the router is getting f7Mb.

Thank in advance for any help.

Nigel

Is the iphone consistent in showing 15 Mbps, and one assumes this is a speed test result.

It is worth running the test at https://www.thinkbroadband.com/speedtest on both the iphone and PC and posting the results link for both, as the differences may be informative.

Does the PC ever see speeds close to the speeds seen by the router, and when you say 17 Mbps coming into the router can we confirm that this is the connection speed shown by the router, and not the speed that the product was advertised at when sold to you.

Also is the PC connecting to the router over an Ethernet cable or a Wireless link? Or HomePlugs?