Why might my internet connection be slow?

I have a problem with my TP-Link TGR1900 (Google OnHub) router. My ISP gives me 250 Mbps. I have tested my service by linking ethernet directly to modem. It is good. At one point after I reset my router, my initial test was around 250 Mbps through the Google WiFi app. But when I tested again, it dropped to 30-40 Mbps. I have:

- reset router

- rest modem

- tried different ethernet cables

- disconnected my ethernet switch

Nothing seems to help. Do you have any good ideas or should I just buy a new router?

Netgear 16-Port Gigabit Ethernet Unmanaged Switch, Desktop (GS316)