ironically l had speed issues a week back, firstly a cable one lol, had a cat 5 to my second router l realized which l upgraded to a spare l had to my second router, then my lan network on my pc decided to revert to a 100mbps link as week later for some reason, went into device manager, network adapters then looked at my settings under speed and duplex it was set to auto dropped it down to 1gbps which got me my 500mbps speed back..
it was weird my speed went to 100mbps, not sure we have the same issue but its worth checking the settings.