Why does making a join layer editable cause so many issues?

I’m using QGIS. When I do a join it creates new column values with a long title because of the prefix it adds from the join. I need to shorten the column names in order to create new shapefiles from different selections from the attribute table. I tried to re-do the join and make the join layer editable, as in the screen shot. It completes the join, but every time I try to open the new attribute table it stalls and QGIS crashes. What happened here?