Slow WiFi Speeds

On my Samsung galaxy note 10, I am getting internet speeds of around 30-40mbps. If I restart my phone or disconnect and reconnect to the WiFi then the speeds will increase to 400-500 Mbps. This last for about 5 minutes until it drops back down to 30. Other devices on the same network have 400-500 Mbps all the time. Wondering if anyone knew what was going on here ? And a possible fix? Thanks.