When you attempt to make changes to a block in the AutoCAD Block Editor, the cursor moves extremely slowly.
In some AutoCAD installations, opening the Block Editor also opens an additional tool panel called the Authoring Palettes. These editing tools seem to make the Block Editor run excessively slowly.
The solution is simple: turn off the Authoring Palettes. Click the Authoring Palettes button on the Block Editor ribbon (which opens automatically when you open the Block Editor).
Once you turn off the Authoring Palettes, you should notice an immediate improvement in the performance of the Block Editor.