A slow data connection on your phone is usually caused by a poor connection in your location, network congestion, or too many background apps running.
Why is Wi-Fi slow on my phone but not my laptop?
This happen because the capability of mobile network system is lower than laptop because it has a mobile cpu and a small ram and a tiny wifi card but the laptop has a bigger cpu more ram and a bigger wifi card . So, It helps in to improve your wifi range and Signal and the Wifi version also important in this factor.
How do I fix slow Wi-Fi on my iPhone?
You may be able to fix a slow internet connection by resetting or reconfiguring your iPhone's network settings, or speeding up your browser app. Otherwise, the problem is likely due to your Wi-Fi — try restarting your router to boost your signal strength or learn how to fix your router if it's not working.