20-30 seconds maybe? Also, it doesn’t happen when renaming other categories. 
edit: also happens on my development forum (fairly strong computer with a ubuntu subsystem).
re-edit:
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splits by smaller batches seem to work. So Discourse doesn’t like to move more than 3000 topics, even of a more powerful server (I upscaled to a 16 vCPU / 32 GB). So I guess that this issue is different from renaming my categories. I still couldn’t rename the category.
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I created a new category B (which I can rename at will), moved all the topics from the category A (which I can’t rename) to the category B: then I can’t rename category B anymore, and I can rename the category A.
I tried to rename my categories, choosing a category with more posts in it each time. Category with more content (more posts, larger topics) seems to take more time to be renamed.
There are 2 categories that I can’t rename (seems to take too much time indeed…):
I can rename Public profile messages, which contain only topics without replies.