Fix performance

[mdvanes]

Note: Run audits against build:pp (with https on http-server -S -C -K)

• [x] convert images from png to (Progressive) jpg and jpg2000 or webp (same image at PNG 313kB, JPG @ 90% 199kB, JPG @ 70% 112kB, WEBP @ 70% 75kB (https://webp-converter.com/)

• [ ] show some content even before javascript is loaded (noscript?)

• [x] lazy load the Post elements e.g. https://www.npmjs.com/package/react-lazy (LazyGroup!) or https://github.com/jasonslyvia/react-lazyload

• [x] serve small images for mobile (in PostPreview and PostCover)

• [ ] gzip does not seem to be an issue on prod

• App scores low on performance audit with Lighthouse. Score 42 with first meaningful paint after 4960 ms

• Without any post previews: Score 72 with first meaningful paint after 2180 ms

• Without any post previews, without mappings: Score 72 with first meaningful paint after 2190 ms

• With post previews, but all the same cover images: Score 48 with first meaningful paint after 4830 ms

• With post previews, but no cover images: Score 53 with first meaningful paint after 4580 ms

• Only normal post previews (not retro), but no cover images: Score 57 with first meaningful paint after 4590 ms

• Only normal post previews (not retro), only Card/CardText: Score 72 with first meaningful paint after 2240 ms

• Only normal post previews (not retro), minus Media: Score 54 with first meaningful paint after 4580 ms

• Only normal post previews (not retro), minus GatsbyLink: Score 58 with first meaningful paint after 4540 ms

• Only normal post previews (not retro), minus CardTitle (w avatar): Score 67 with first meaningful paint after 2450 ms