Improve website performance

[itrich]

We should improve the website performance and gain a performance index of at least >90 at https://web.dev/measure/.

Some ideas to accelerate page load:

• Reduce image size with a corresponding jekyll plugin (e.g. valerijaspasojevic/jekyll-compress-images or envygeeks/jekyll-assets)
• Minify documents and text-based assets
• Make extensive use of lazy-load (especially on long pages!)
• Investigate benefits of using a CDN (see discussion Move website to a CDN? #300)

Any further suggestions?