Clarification on when this is available/applies:

Preloading extensions on background thread began in version 18.0, and is now enabled for 50% of developers. Starting 18.4 we will bring it to 100%. Also, this experience is limited to reopening a solution, e.g. from the “Get started” window or “File > Recent Projects and Solutions”. Thank you for your feedback on how much details you expect to see in the blog posts!