To get crisp images on hidpi displays, a common approach is to load an image double de dimension and rescale it down. E.g., load an 200x200 image and rescale it to 100x100. In Emacs this can be done by

(insert-image (create-image "200x200img.png" 'png nil :scale 0.5))

And Emacs will display a crisp 100x100 image in the buffer. However, this doesn’t work for sag images. If I do

(progn
  (require 'svg)
  (let ((svg (svg-create 200 200)))
    (svg-text svg "test"
              :font-size 100
              :y 150)
    (insert-image (svg-image svg :scale 0.5))))

The image is still blurry: