I don't know what I changed if anything, but new files I open have a png transparent like scratch area whereas it used to be a single solid grey colour all around. The new grid scratch area is somehow slowing performance and making working on my projects harder. The artboard also has a series of green almost guide lines but they're permanently affixed to the artboard and I can't move them. Is there something I accidently changed?
You've created a new file based on a Film & Video document preset, which turns on a number of view options like safe areas, crosshairs, and the transparency grid. You can turn off all these things one by one, but the quickest way to get what you want is to use a different document preset.