Large Pages document grinding to a halt, won't save
I have a non-iCloud Pages file that includes images and other dynamic content that I've edited without incident multiple times over the years in Pages. It's a 13MB file.
Now, in Pages 5.6.1 (2562), I opened the document, selected to update the last edited date feature, then it became unresponsive. It pegs one or two processors on the machine (MacBook Pro 13-inch, Late 2011, with 16GB memory on 2.4 GHz Core i5 running El Capitan 10.11.4). I can't interact with the document.
I've left it open overnight and it's still running at 100% or 200% sometimes, only adding an error that it can't autosave. I tried closing it multiple times and ended up force quitting Pages out of impatience. As soon as I reopen it, it hangs again. Today, I tried closing the doc again, and was able to press the red X to close it. I clicked for it to save changes but, after Pages closed the doc, it remains unchanged since Mar 19.
Since closing the file, I've tested other files and they seem ok.
I'll try to open the doc again and see what happens, but I was unable to use Pages all of last week because of this. Any ideas?
MacBook Pro (13-inch Late 2011), OS X El Capitan (10.11.4)