O.K, I spent some time with the code and think I found the cause for the slow loading.
It looks like the cause is the nlohmann/json library.
When loading the XML version, it loads up in 1 second where in the json version it takes around 10 (!).
Also it is not clear to me why the XML spriter file is smaller than the JSON, since JSON should be a'lot thinner, but thats another question.
Lucid, I assume that when you checked it yourself, you did that with the SCML version too, right ?