Word – Really slow editing of a big document, but computer has resources unused

When editing a .docx of 423 pages with text and pictures, every small editing is really slow. As an example, I have measured how long it takes to remove the borders of a text box and it lasted 2 minutes 11 seconds. While doing this winword.exe*32 was using 25% of CPU and 490216KB of memory. But the overall usage of the computer was CPU 26% and Physical Memory 31% hence there were plenty of resources to be claiming rather than being this slow.

I have searched here and all I found is Word (2010): There is insufficient memory or disk space. Word cannot display the requested font and I'm not seeing any message from Windows or Word.

What can I make to speed up the editing of the document? Is there any way to force Word to use more resources even if they're not needed at the moment? Will this help?
Thanks,

EDIT: I've been asked to explain why I consider this question not a duplicate of Large document is sluggish and its pictures not visible in Web Layout

• that question has lots of advices I will follow, just in case, so anyway thanks for point me out this (sluggish is not a word I knew, so I missed this question)
• However, they talk about 2 seconds slow, Oh man I wish I had this problem. I'm talking about 2 minutes and a bit, more or less 60 times slower.
• Also I cannot use Word 2013. I'm bound to Word 2010.
• I was in Print Layout from the beginning.

As I said, I will start one by one with all of those advices and write here the progress, thanks for the link.

1. I tried on another computer (original: Windows 7 64b – 16GB RAM – i7 – Word installed in Spanish, new: Windows 10 64b – 12GB RAM – i5 – Word installed in English) and it went from 2 minutes to 55 seconds.

2. Installed DataRAM RAMDisk (freeware) and placed the document on a newly created, 768MB NTFS drive. 1 minute 52 secods.

3. RAMDisk AND Disable "autorecover every 10 minutes". 1 minute 52 seconds.

4. Save As in "Word 97-2003 Document (*.doc)" format => It says 'shapes and textboxes will be converted to effects available in this format' with 15985 occurrences. ==> WOW! Less than 5 seconds! (2 seconds, I wasn't prepared for being that fast)

I don't know what I'm missing from the docx to doc, but if it's not terrible I will keep with the doc format, at least it makes usable to edit it.

• Ummmm now I have just received a message telling me that given insufficient memory Word had to be closed. I still remain with option #4 and will save on a regular basis.

FINAL EDIT:

Finally I have done it using the conversion from docx to doc. What have been lost is related to bullets and layout (15985 occurrences!) but this is not important for me since I had to change the format anyway.