I recently discovered a way to help people who are experiencing performance problems opening and saving large Excel workbooks over a WAN connection.
The solution is to switch to a more efficient (smaller) file format. So instead of saving the workbooks as XLS or XLSX, you need to save them in XLSB format.
The "B" stands for binary, and it means that the file is saved in an optimal binary format, as it was before the trend to use XML to make file data more "open". So the files would be harder to read in other applications, but much faster to read by Excel.
XLSB format supports all of the functionality of the other file formats, including macro and VBA code.
And because the files are much smaller (down from 10 MB to 2 MB in one case I tested), they can be read/written over a WAN connection much more quickly.
One thing to watch out for, of course, is that you will be changing your filenames. So if you're using external links they you will have to reconnect them to the new filename. External links work faster too, of course.
Note: They may also be a problem with "Custom Ribbons" and "Personal.XLSB" which you need to watch out for but I have not experienced this.