Wi-Fi Assist is a feature on iOS devices that automatically switches to cellular data when it detects a weak Wi-Fi signal. However, this can sometimes lead to slower speeds if you're not in an area with strong cellular coverage, so it's better to disable this feature. To turn off Wi-Fi Assist, go to Settings > Cellular and scroll down to the bottom of the page where it says "Wi-Fi Assist." Toggle the switch off and see how it improves your speed.