Background images are not usable if not compressed or breakpoint-versioned.
It's possible that we may build out this functionality one day, though it's unlikely that we'll build this out in the near future.
There are many resources out there which you can use to resize your images other than image optimizing/editing tools like Photoshop or Lightroom.
Here are a few free image optimizing tools:
Does the current background image create versions (smaller version for mobile? as i presume a regular image block does?
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