Hello Lauradig,
Just to clarify, is the issue that the Messages app stops responding when trying to scroll? Do you receive any error when the issue happens? If so, what does the error say? Does the issue happen with other apps, or does the entire device seem to stop responding?
The above information will help us determine how to proceed.
If your iPhone is running slowly, please check out this link, which has some information that may help with device performance: If your iPhone or iPad is running slow
Check your network conditions
Many apps require an internet connection for their content and other functionality. If the network that you're connected to is congested — for example, at an event with many people using the same network — apps might take a while to open or display their content. Your device can also seem slow as it repeatedly reconnects to new cell towers when you're in a moving vehicle.
Even if your device indicates a strong signal, you might need to wait, try again in a different location, or use available Wi-Fi. If you continue to experience issues with cellular data speed, availability, or performance in multiple locations, contact your carrier.
Learn more about cellular data >
Thanks!