If you are using an iPhone or any other Apple device and you select multiple photos or videos, you may experience a delay when you tap the "Add" button. This delay occurs because Apple needs to convert the files from a format called HEIF/HEVC to a more compatible format for the web. The conversion process can take up to 1 second per photo and up to 4 seconds per video.
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If your phone's camera is not using the HEIF/HEVC format, there should be no delay with your photos and videos.