Game patches after version 1.25 introduced severe stuttering in the Steam version of game.  This issue is not present in the Microsoft Store or Origin versions of the game.
In order to solve this, you must roll back the game to version 1.25. Savegames saved on any update after 1.25 cannot be rolled back, and will not work on version 1.25. However, existing saves will not be modified by the rollback process. Updating the game renders them playable again (albeit with the same stuttering issues). Updating the game also does not, by itself, modify existing saves. So long as you don't overwrite any save made prior to version 1.25, they will remain playable upon rolling back again.
Be warned that DLC content "Episode Ardyn" will not be accessible from version 1.25. You may however play the entire game, including previous DLC episodes, up to Episode Ardyn and then update the game. It is recommended to make a back-up save on a different save slot before updating the game, so that you may go back to version 1.25 and continue playing the game up to before Episode Ardyn without performance issues.
Final Fantasy XV uses heavy resources on a PC's paging file, which acts as alternate virtual RAM.
If the paging file is set too low (such as a maximum of 2 GB), this can act as a performance bottleneck, resulting in frequent stutters while exploring. This is especially true if the system's RAM is 8 GB and under. Increasing the paging file size (such as from 2 GB to 8 GB) can drastically improve performance and reduce stutters.