Wifi on MAC really slow until I reboot

I have a Mac with Mojave 10.14.5.

I have a problem with the home Wi-Fi. I have a 400 Mbps connection. When I start the laptop, the Wi-Fi is fine, but eventually it drops to less than 100 Mbps, and this somehow affects all other devices (phone, TV) that are connected to the Wi-Fi network.

But shortly after I restarted the Mac, the speed returns to 400, both on the Mac and on the other devices.