600 download but still lagging help

We just upgraded from 275 to 600 first we had an issue because we didn’t get what we should even on land they came and fixed it by changing the hz from 2.5 to 5. But now when I’m gaming even with land on my pc I’m lagging going from 50 ping to 400 but then I go and check my internet on Speedtest and shows 600+ download plz help