Space export progress bar is inaccurate

Symptoms

When exporting a space, the progress bar usually gets stuck at 0% or 99% for nearly the whole time, up until when the export is complete. At the end, it suddenly goes to complete. The running time updates normally. The export still works fine, producing a file, but it may look like there is no progress being made.

Steps to Reproduce

1. Find a space which includes a lot of data and/or attachments
2. Go to Space Tools > Content Tools, then do a full export to XML
3. Watch the progress bar as the export is made