Sometimes, what we perceive as "slow internet" is actually just a weak WiFi signal. Due to interference and signal loss over distance, wireless speeds typically perform worse than cable rates on the same network. However, there are techniques to guarantee that your WiFi is operating as quickly as possible.
Optimize your router. See #1 above.
Reduce interference from other electronics. Other devices in your home can slow down your WiFi connection, including microwaves, cordless phones, Bluetooth devices, TVs, wireless security systems, baby monitors, garage door openers, and more. If you have a newer modem, opt for a 5 GHz frequency signal to get a stronger connection and avoid some congestion from surrounding devices, many of which use the 2.4 GHz band.