Disable Big Image Scaling

Stops WordPress from creating a scaled-down "-scaled" copy of images larger than 2560 pixels, so the original file is used as uploaded.

Code

add_filter( 'big_image_size_threshold', '__return_false' );

Very large originals use more disk space and can be slow to serve; resize before uploading.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.