HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Most likely, you have a rendering setting (renderer) and stretch filter that are too demanding on your processor. Try experimenting with these settings and see which option is more efficient. It's also possible that your window is at its maximum size for a FullHD or even 4K monitor. You can reduce the window size.
(My screenshot is just for illustration, to show "where to look," and not "recommended settings," especially since this is an older version of the HD mod).