Yes exactly. If the category is excluded, then all the posts are eleminated in that category from the results. There is a setting to change that to every category instead of any category, that is what you need. I already switched that setting for you. (general options panel -> behavior -> terms logic -> OR)
This results only showing posts from the excluded category 😉
Yes it indeed slows down, because on the server side it actually does the opposite – it matches the posts to eleminate with the excluded categories. However it's fixed as well with the upper mentioned option.
It should return posts now from only the Baby Sign language category with much better speed.Best,
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