I'd like to see some analysis of the psychological side of this. I'm not convinced these sorts of previews/placeholders are a positive thing, I find most of them rather distracting. The SVG approach is fine as a novelty, but to assume that this is the best approach from a user point of view is quite a big assumption. My intuition is that the hard edges of SVG are more distracting (attracting your eyes to something that isn't going to give them meaningful information), but that a blurred image messes with your eyes a bit more. I wouldn't really like either of them to be used on the majority of sites.
I prefer something very subtle to show that there is an image that is supposed to be there. If it is going to show a single color rectangle, I prefer it be translucent so as to attract even less attention.
Anecdata: In a newsfeed-type product at our company, we implemented content placeholders that communicate loading state instead of a traditional loading spinner. This reduced user drop off significantly, and bought us some time to work on the real problem under the hood which was latency.
Aside from my anecdote, there are many blog posts and experiment results out there that suggests that this works, and it works well.
Also I have suspicions about what you are measuring being 100% correlated with "better user experience." Lack of user drop off CAN indicate a better user experience, but the concept of "click bait" illustrates that you can gain short term increases in attention in ways that both decrease user experience and can contribute to driving away users in the long term. I'm not saying this sort of thing is the same as click bait, but still. I'm sure that in theory, you could put lots of things in those placeholder spots that will increase the number of people that wait for the page to load, but may drive away users in the long term. (e.g. blurry nudes)
I prefer something very subtle to show that there is an image that is supposed to be there. If it is going to show a single color rectangle, I prefer it be translucent so as to attract even less attention.