Why might my internet connection be slow?

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: