Sometimes Word will freeze, or slow down, because the file size it too large. If this happens there are a couple of ways to troubleshoot the issue.
First try clearing all formatting from the document.
Both techniques will clear all formatting from the document. You can then start to apply each layer of formatting step-by-step.
When it comes to images, alt text shouldn't affect anything, but it's possible the images are really big.
You can try deleting all images, re-saving them in .jpeg format from the original, and re-inserting into Word (Insert>Picture).