slow internet on macbook air

I have a Macbok air, 13 inch early 2015 running 10.11.6. fully updated. On this machine browsing can be very slow. However other Macs on my network, including one on the same desk and accessing the same access point work at good speeds. I have run my ISP speed checker on two macs simultaneously and the slow machine reports similar values to the other machine - so I do not think it is my wireless network.

I have tested the slow mac on an ethernet connection but find the same slow page load speeds

I have checked the DNS settings and deleted a long list to restore the usual default values that work well on my other Macs without any improvement. All the other settings match those of my other Macs.

I have loaded Firefox as an alternative browser but find the same problem - my download speed was good, it seems to be a problem loading page data.

Any thoughts greatfully received.