The pattern
Speeds are great at noon. Garbage 7-10pm. Same router, same devices, same physics. Something else is the bottleneck.
What is actually slow
Not your Wi-Fi. Your ISP’s local node serves dozens of homes, and at peak hours everyone on it is pulling video. The pipe to the rest of the internet is shared and contested.
What might help
5GHz instead of 2.4GHz reduces interference inside your home but does not fix ISP-side congestion. Fiber, business-tier service, or a different ISP does.