When you find your WiFi signal slow and lousy, don't hurry to buy a WiFi extender immediately, you should test your WiFi speed first and think about a proper plan according to your WiFi speed and your demand for using WiFi signal.
Another way to test your WiFi speed is to use a tool like inSSIDer. This tool will show you all the networks in your area and their signal strength. The greater the signal, the faster the speed.
You can also use a tool like
NetSpot to see what channels are being used by the networks around you. If there are a lot of networks using the same channel, it can slow down your speed. So, if you see that many networks are using the same channel, you can try changing the channel on your router to see if that speeds things up.