Can you try this:
With the book open in Atticus, click on the Formatting Tab, and select Print from the dropdown menu over the previewer.
Select your Table of Contents from the left navigation pane and allow this section to render in the previewer. Note: the TOC will be the last section to render in the previewer, so depending on the length and complexity of your document, this may take a few minutes.
After the TOC is rendered in the preview, export a fresh copy of the PDF using the export button under the preview.
This will allow Atticus to have everything pre-rendered before you export the PDF file, which will ease the burden on your browser cache and speed things along during the export process.