Linux Mint 18. Presentation is about 38 pages. Files in Writer and Calc save immediately. Impress just sits during the save process, the completes any subsequent keystrokes.
Number of pages isn’t a good measure to assess your save time. If each of you slide contains an embedded video of 5 mins length, then 3-4 miniutes seems reasonable while 38 pages with 20 words of pure text doesn’t.
If you don’t have large videos there is probably a problem with your file. Do some testing to locate the problem by deleting slides in a copy …
My 40 slide presentation with full images on every slide is 17 MBytes and saves in 4 seconds.
I had previously optimised the presentation with Tools > Minimise presentation.
A 150 slide presentation with 150 large photo images (each 4,000 x 3,000 pixels) and not minimised is 243 MBytes and saves in less than 1 minute.