@Clorilda 

There is a possibility something produces wireless signal interference that impacts speed of your WiFi network. On restart, Decos will rescan WiFi channels and switch to less congested.

Next time, use "Network Optimization" in Deco app instead of resetting Deco mesh, and see if it brings back WiFi speeds. That won't fix the underlying issue, but could help to narrow it down. 

I would also recommend installing WiFi Analyzer app on smartphone and use it to check how congested your house is with WiFi networks from neighbors. I prefer WiFiman from Ubiquiti, but there are other similar apps, too.

@Alexandre. 

I've already tried using optimization but it doesn't work.

Only rebooting solve the problem.

And sometimes it needs 2 or 3 reboots to work.

I'll try the app when it happens again.

Thanks for de reply.

Does anyone else have any suggestions?