Unable to scroll in Pages app

When you state that "it works fine in other apps," presumably that is scrolling and tested in other applications at the same time that scrolling in Pages is sluggish, not some protracted time later.

The issue at hand is whether some other process on the Mac is maximizing its CPU usage causing wait states in Pages, or that your Pages document is on iCloud and the current network connection to that document is abysmally slow. Either will impact observed scrolling behavior. Same story with a good network connection but nearly exhausted iCloud storage.

You haven't stated how large or whether the Pages document is full of high resolution images. That will make scrolling lethargic. If your scrolling observation is right after powering the Mac on, you may be competing with Spotlight's dozens of indexing tasks and that will slow everything down.

There can be other factors but these are contributors to slow scrolling. Hopefully, you are not running any third-party anti-virus tool that can usurp normal operating system and application behavior.

Delete the Pages app and redownload it from the App Store.

Try restarting your device or force-quitting the Pages app to see if that helps. If the issue persists, you could try updating the app or restarting your device.