[Q] wifi slow and laggy topic

Wifi on my HTC One seems to lock up, stutter and pictures etc are slow to load.
If I turn off Wifi and back on everything is fine for a couple minutes.
Then things slow down again.

I restored my phone (google backup was on though), replaced the router (with the same router).

Everything worked fine up until 2 weeks ago.

Wifi on other routers works fine.
Could I have old cache somewhere?