for mac it's FSEveents: https://developer.apple.com/documentation/coreservices/file_....

> As soon as I paid for the Plus Plan, I started having to wait several minutes for a file to upload. Even from one computer to the other with LAN Sync enabled. The day before that, my files would sync faster than I could hit refresh in the browser I was developing in.

that definitely seems like a bug, could you report it? click on the dropbox tray, click on the dropdown in the top right, and then click "Report Bug." this will collect some information about your client to help us debug.