> but is there a way to set this behaviour as default?
A default behavior can be set for images which are newly imported into the current document (via Inkscape's GUI).
There is no feature to re-interpret the default rendering mode ('smooth') if 'image-rendering' presentation attribute or style property is unset (the default fallback is defined in the SVG spec), or to override the attribute globally (if 'image-rendering' is set to 'optimizeQuality' for individual images), nor is there a built-in command to add the non-default rendering mode attribute to all images in the current document, or to change it to 'blocky' ('optimizeSpeed') for a selection or for all images in the current document.
> but is there a way to set this behaviour as default?
A default behavior can be set for images which are newly imported into the current document (via Inkscape's GUI).
There is no feature to re-interpret the default rendering mode ('smooth') if 'image-rendering' presentation attribute or style property is unset (the default fallback is defined in the SVG spec), or to override the attribute globally (if 'image-rendering' is set to 'optimizeQuality' for individual images), nor is there a built-in command to add the non-default rendering mode attribute to all images in the current document, or to change it to 'blocky' ('optimizeSpeed') for a selection or for all images in the current document.