1) You say “Localized Error”. If you believe that there’s an error, you should just report it in the Support forum, kindly waiting for the developers to analyze and, in case, provide the fix.
2) Without entering into technical details, “slowness” in saving or in undo’s can depend on many factors, none of which is basically in our control. To give you an example, if your page include big or many images then your browser needs more time to process them, depending also on the computer or device you’re using (a powerful desktop computer is often much faster than a tablet, etc.).
3) In any case, thank you for your feedback, and we wish you could appreciate and rate Zedity for what it enables you to achieve, not for the time your computer needs to save or undo your work.