@yukinoh1989 O.k., after 6 failed attempts to upload it to Dropbox, I decided to try Google Drive and that worked.
Let’ see if you can open this: https://drive.google.com/open?id=1q0Rhn7Blf_mZvKDFi-9w-V8xIQU52Sk8
The only change I made, was to remove the Multires and at least for me it opened a lot faster!
From about 60 seconds for the original .blend to about 15 seconds for the new one.
Like I already said, your poly count is not too bad, apart from ‘Sphere’, which has a Subdivision Surface Modifier with a Viewport level of 3! Change that to 2 and the total amount of vertices will be just over 600 000. (And then it opened in about 10 seconds…)
So, I would say that there is no need for decimation.
