Hi Reier!
For starters refresh your map screen (bottom right corner).
Next open your task manager and see how much memory you're using.
Check to see if your computer meets system requirements.
https://pro.arcgis.com/en/pro-app/latest/get-started/arcgis-pro-system-requirements.htm
Sometimes all you have to do is refresh. If your memory is maxed out, turn off or remove the DEM from your map. Then save project. Close down unnecessary programs or browser tabs to free up memory.
Raster processing and display uses more memory than vector processing.
https://pro.arcgis.com/en/pro-app/latest/tool-reference/spatial-analyst/slope.htm
Let us know what you found out.