That depends entirely on how much data you are saving. Your best bet is to test and see if there is an impact. Nothing DefSave does should have more of an impact over using the features it uses directly, it is purely a convenience tool.
If you are saving small changes in large files then it would be better to save those small changes in their own files if possible.
DefSave’s concept of “files” internally puts everything into one big blob if I remember right so you would want to save more often saves into smaller instances of DefSave probably. I’ll read the source in a bit to check… “Files” should be separate in DefSave as far as IO goes so you should only need one instance of DefSave for multiple files.
If you want absolute fastest performance you would need to make a custom solution for your data types with low level IO.
I don’t think built in file IO is async, may be worth someone making a native extension which can do that.
tl;dr test and see if there are issues. With flash memory it should be too fast to notice for small data even with no async IO.