Hello Jorge,

maybe it takes time just for the first time? 

Please make sure that in your v9 project every resources (photos, videos) were visible in every page of your preview

In order to verify that, you need to open the v9 preview while holding down the CTRL key of your keyboard, it will take some time but every single resource will be recreated. If you miss some resource, and this could be the reason of the slowness on the v10, is because the original file was removed from his place and you need to place it in the same path again or import it again from another path.

Keep me updated, thank you