There are usually some third-party applications that bring useful functions such as slide typing on an iOS keyboard.
 

How to Use Scroll Writing on iPhone

Select the keyboard application that supports fast-scrolling, and download it from the App Store. (Here the Gboard application was selected).

Open Settings, drag down if necessary, and press the Keyboard.

Click Keyboards at the top, then select Add New Keyboard.

Now, press the Gboard keyboard in the lower menu.

Privacy

You can click on the Gboard and turn on / off full access. As shown below in the image on the left, each third-party keyboard will have access to what you type, but turning off the "Allow Full Access" option means third parties will enforce Apple's terms and conditions if they collect or send data. 

 To use the drag / drop function in the new keyboard, click the globe icon in the lower right corner until you see the new keyboard that is already installed.