The best way to "download better" is to ensure you are getting legitimate, stable images rather than unstable "hacked" versions found on community forums.
Shrink your images to save disk space using the qemu-img convert -c command. This can significantly reduce the footprint of Windows or large Linux nodes. eveng qemu images download better
This is the gold standard for Cisco images. A CML Personal subscription provides legal access to IOSv, IOSv-L2, ASAv, and NX-OS images that are optimized for virtualization. The best way to "download better" is to