When gathering files from a content source, the SharePoint 2013 Crawl Component can be very I/O intensive process – locally writing all of the files it gathers from content repositories to its to temporary file paths and having them read by the Content Processing Component during document parsing. This post can help you understand where the Crawl Components write temporary files, which can help in planning and performance troubleshooting (e.g. Why does disk performance of my C:\ drive get so bad – or worse, fill up – when I start a large crawl?)