The Documents Tabs may not be relevant to your time issues.
There are some things we can assist in the design file and the others are by you on your machine.
Machine first.
Are you running the 64-bit Xara Desktop application or an earlier 32-bit?
If 32-bit then your computer memory is constrained.
If 64-bit and you have only 8GB RAM then you need to close as many other applications as possible. The second Document Tab (Coundowns) could be saved and closed but this will only help if the file is also very large.
You can check through Control Panel > System and Security > Task Manager: Memory column against your Desktop application.
I have to assume your "only updated pages" is a mis-remembering of Web Properties > Publish Settings > Fast Publish (Only changed files).
Have you used Utilities > Optimise all images > Settings and, changed the resolution and pressed the Optimise button? This tunes you images to be adequate for web viewing.
Ensure you have your master images already in a safe area.
You can test the effectiveness of the above by Exporting to a local folder and checking the overall size of the ../index_htm_files/ folder before and after.
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