Downloading games is only quick after restarting the WiFi router?

Hello, since today I've been having a problem where downloading games and apps has been slow until I restart the Fritzbox. After that, the download speed is fast again. To be precise, when I download something, it downloads at 500 kb/s. After I restart the router, it downloads at 10 mb/s. But if I download something else a few hours later, it's back at 500 kb/s. But it always works again with a restart.