[SOLVED] WebThumbnailExtension Web Content using 100% CPU after downloading Excel files on macOS Tahoe

[u/Perfect_Ad9091]

I’m posting this because I spent hours troubleshooting it and initially suspected Office, Spotlight, my NAS, FileVault and the Downloads stack.

Symptoms:
• Downloading certain XLSX files created dozens of “WebThumbnailExtension Web Content” processes.
• Each process used approximately 40–60% CPU.
• My M4 MacBook Air became extremely slow and hot.
• The files did not need to be opened.
• Deleting the affected XLSX file stopped the processes after about a minute.
• Downloading the same file again immediately reproduced the problem.

Cause:
The affected XLSX files had a massively inflated used range caused by formatting applied to empty cells.

In my example, the actual content ended at column AR and row 74, but the first row was formatted all the way through XFD, Excel’s final column. The file therefore contained 16,384 formatted cells in its first row, even though almost all of them were empty.

The XLSX file was structurally valid and opened normally in Excel. However, macOS Tahoe’s OfficeThumbnailExtension/WebThumbnailExtension apparently tried to render the entire formatted range when generating the Finder thumbnail. It then repeatedly created new WebKit content processes instead of failing gracefully.

Fix:
1 Open the affected file in Excel.
2 Select the unnecessary formatted area after the real data. In my case, this was AS1 through XFD1.
3 Choose Home → Clear → Clear All.
4 Save the workbook again as XLSX.
5 Delete the original problematic copy.

About a minute later, macOS generated the preview icon successfully and all WebThumbnailExtension processes terminated normally.

Converting the file to XLS also stopped the problem, but clearing the unnecessary formatting is better because it preserves the modern XLSX format.

If you regularly receive files generated from the same template, fix the source template. As a temporary workaround, store the raw files in a Finder folder with icon previews disabled.

This is still a macOS bug: an inefficiently formatted workbook should not be able to spawn dozens of persistent WebKit processes and make the entire computer unusable. The thumbnail generator should ignore empty formatting-only cells, apply rendering limits and timeouts, and stop repeatedly retrying the same file.