What router do you have?
How many wired devices are connected to it?
how many wireless devices are connected to it?
Do ALL devices slow down or just specific devices?
What band(s) is the router CURRENTLY putting out?
Is this problem regardless of the time of day or during peak consumer times (ie mid afternoon to just after sunset)?
Have you ran any speed tests to discover "Exactly" how much your speed changes? If not, use Ookla's speed test as opposed to your ISP's speed test, and run multiple tests and do these at various times if possible. Internet speeds fluctuate, especially during peak times.
Do you have any other wireless outputting devices (Routers and similar devices) in your house?
Does your home utilize more than one internet modem?