I'm taking from experience. try it yourself.
Open a word docx file and change one letter. save. It should save before you blink, no mater how big the file is. Now open a epub/azw3 in the calibre editor. Change one letter and save. The bigger the file - the longer it will take.
If your still not convinced, do the same as above and have the file browser open the same time. By the docx you wont see any deference. But by the epub you'll see a second file, with a random name, appear temporarily.
The only practical difference (besides the time) is if the editor hangs in middle of saving (because the file was to big
). A docx file will be partially saved. A epub will have the original and half of the new file (probably corrupt).
BTW when i said big i meant 200 mb = at least 2 minutes to save, no matter how little i edit.