The thumbnail.agent is an app that (I think) goes into a file, reads it, and displays its content in thumbnail form. It runs within Finder, obviously. This happened to me because I had a huge pdf on my finder, which thumbnail.agent continuously looked into and updates (quick fix would probably be if Apple devs made it run every 10secs not continuously), a more decent fix would be to generate a less accurate thumbnail so it uses less resources. User Solution: The moment you switch to another window in Finder where there are no big files you’ll be fine. I host my big pdfs online now and open them in my browser to prevent this from happening. If you had to have them saved locally you could save them zipped or something. Hope this helps!