While you can try the latest firmware and as close to factory settings the awful truth is that for high speeds not only must you have a compatible WiFi card but I find I have to install Gigabit dual band routers.

The speed also drops with distance so you will get clients that want full speed at all locations in the home. For them you move to mesh networking.

https://www.google.com/search?q=D-Link+DIR-645+slow+wifi shows this is not an uncommon complaint for this model so after you try new firmware and settings, exchange it fast.