If this is a fairly recent development three things to check on are:
RAM
Free hard disk space
File fragmentation
If all these are within acceptable parameters take a look at
Tools>Options>Save and do the following as needed:
Allow Fast Saves - unchecked
Make local copy... - unchecked
Save AutoRecover info every: - no less than 5 minutes, preferably 10 or more
Is this the case with *all* files or just certain ones?
Are you using Track Changes?
Is there much content other than text (graphics, tables, etc.)?
Are there any other process running when you experience these lags?
What else can you offer in detail?