I excluded a bookmarks book during the import, which means that in the end, I have about half of the notes and labels indicated, but it's still a lot 
Yes, Joplin is fast most of the time (I have a random problem that happens about once a day but it may not be caused by Joplin): barely a second to create a new note or to display the content of a note when I click in the list.
Around 3s when I switch from one tab to another with the Note Tabs plugin, but that may be due to the plugin 
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"Quick" navigation (CTRL + P) takes longer, often at least 10 seconds to display results, but sometimes I use other, faster methods to find notes.
In short, I can breathe again thanks to Joplin 