I had this same issue myself last week, and found that some of my WiFi channels in use were clashing on channels that nearby neighbours were also using. You can do some scanning using free Windows or Android apps (not iOS, but Linux also has a nice app called LinSSID, and some routers will do their own scanning which is sometimes called site survey) to determine which channels are the most open where you are, and then set your router to use one of those instead. The 2.4 GHz segment is especially crowded nowadays (but unfortunately it has the better range). The 5 GHz has many more available channels which are open, but the thing is the range is quite a bit less.
It seems that many routers, which have auto scan ability, tend to go for channel 1 or 6. So often channel 10 upwards will be a bit clearer. But there is also some overlap across channels, so a strong channel 1 and channel near each other, will still interfere.
See Wi-Fi signal interference test — simple trick to speed up your connection
If your Wi-Fi is slow, the problem could be signal interference, which is relatively easy to test for and fix.