Unfortunately disabling background saves is neither desirable nor a complete solution to the problem. The same problem re-occurs in Word 2016 and appears to be one which MS can't replicate and which therefore doesn't exist. perhaps if they tested this on
a slower machine with a slower internet connection they might find it does exist.
I find the problem occurs when a document is created using an older version of Word and is edited in Compatibility mode. Presumably when saving Word checks the document to ensure it will still be compatible with older versions and this makes saving a long document
very slow.
The obvious answer would be to convert the document to Word 2013/2016. There is an option to do this but it doesn't solve the problem.
I find the answer is to copy the entire text and paste it into a blank document created in Word 2013/2016. Once saved that does solve the problem with slow saves and you still get autosaves backing up your document.