Thank you for reporting the issue to us.
We have fixed the issue in the latest version of InDesign, Id 14.0.3
Please upgrade your InDesign application to the latest version to try out the fix.
If you are unable to see the update yet, go to Creative Cloud desktop application, open the menu (three dots) from the top-right corner and click on ‘Check For App Updates’.
If you face any problems with the new update, do let us know in the comments.
You may also follow the below given link for the list of all fixed issues in Id 14.0.3
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JUST UPGRADED TO OS X MOJAVE, and now InDesign has slowed down, especially 1) opening a document and 2) saving a document. Is Adobe aware of this?