hi @Geeza I have done some testing and it is really bad. 9.04s to finish Spotify load.
As you have suggested it is Spotify that cause long load. Because Im not expert on implementing Spotify iFrames here is only my first impression suggestion as your content rendereing is blocked .
In Chrome and Firefox is natively implemented/supported lazy loading but not in Safari
So you can read these articles
Basic informations
In this article is section How do I handle browsers that don’t yet support lazy-loading?
But I believe that you can find more on internet
Good luck