A WiFi analyzer is a small app that shows every wireless network around you, what channel each one is on, and how strong each signal is. Used properly, it can fix slow WiFi without spending a rupee on new hardware.
Some sources don't show up in WiFi analyzers but still cripple performance: microwaves (2.4 GHz), USB 3 hubs leaking RF (2.4 GHz), poorly-shielded HDMI cables, and certain LED light fixtures. If your WiFi mysteriously dies at the same time every day, look for a device that turns on at that time.