Probably time to delete the install and fresh download it again.
This game, like many other Blizzard titles, uses a CASC file system within the game files.
That system can fragment and cause issues over time as you get updates, etc. There is no way to “defragment” that system. You can always try a scan and repair, that might help. You can do that from the launcher, by clicking the gear cog next to the play button and choose “Scan and repair”. The other option is to remove the game folder and re-download.
The best way to do this:
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Exit the game and the Battlenet Launcher. (make sure its not running)
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Navigate to the installation folder for the game itself. Default is
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C:\Program Files x86\Diablo IV.
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Delete the DATA folder in the Diablo IV directory.
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Restart the launcher. You can then update/install the game again. (it will download a fresh copy of the game)
Good luck.