Your HD appears to be almost empty with plenty of free space.
Fragmentation would not seem to be an issue. Cluster size  4k, are
you using FAT or FAT32? Based on the size of the drive, I would
guess FAT32. There are limitations using FAT, FAT32 should work,
NTFS would be better.

What version of Windows, Excel, How much installed RAM?

Is it possible to duplicate the problem on a different machine?

I have followed some of your messages. You mention the use of png's.
Have you considered temporarily removing them? Or perhaps using a
different format for your graphics.

I am not an expert on Excel but have repaired many PC's.