I agree with the responses to your query which are quite accurate. To rule out its a mobile phone or carrier issue check if other people in your household with mobile or any other mobile you have has the same issue. If yes then there might be some interference caused due to microwave, radiowaves etc with the Wi-Fi signal. Also, if the mobile phones are old technology then there might be some compatibility issues. However, if it’s only your mobile then there could be some hardware or software issue on your phone that might be causing slow WiFi. Ensure you upgrade to the latest version by going to the necessary website. For hardware issue probably a firmware download could solve the same otherwise you might have to get a new device.
Also, applications and programs running in the background on your mobile could be hogging most of the bandwidth. One solution is to download a bandwidth management software and prioritise which apps get how much bandwidth. This might increase your slow Wi-Fi connection. Also, increase the memory of your device by clearing cache and other unwanted data saved on it which also slows internet or Wi-Fi connection.