Thank you all — Levon kramwe, and guys from the wonderful bubble.io project – with your comments I could improve the whole speed thing to a real impressive outcome, I did not even expect when I started optimizing.
1. Cloudflare (caching the files in the network) and as in post said. Also Argo is increasing - again - response times by > 60% at my site.
- next to cloudflare, do not forget adding PWA elemnts (caching on the browser side)
The outcome was finally perfect score in Google Lighthouse Tests with almost 100, also on Mobiles.
Then in page rules I did set “cache everything”, for all secondra.com/* pages. I would recommend a thourough test on your app with any adjustments in Cloudflare, with a documentation (I did it in a simple spreadsheet).
For me, there is no disadvantage in using bubble anymore, and I think these caching-optimizations has to be done in any development framework one chose. So it is the work of the “Webmaster” if you have one. In most bubblers corporate situation it is me & you, haha. 
