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Where did you read "...about a reduced speed (for access) when the limmit is reached" and when was it written?
I've been using calibre for almost 7 years, my perception is that its gotten faster over time. It's about the same on my current 16GB, i7-7700 as it was on my old 6GB i5-2300 with much slower storage devices and graphics. The only time I ever noticed it was slow was when I put it on a Dell Alien Alpha gaming box, but so was everything else
My biggest library just topped 150,000 books, I have other libraries between 100 and 8,000 books, performance is about the same on all of them, including start up times which is sub 10 seconds. Scrolling the list is pretty much instantaneous, as is searching and virtual library switching.
Tools that access the ebook file contents - e.g. conversion, plugins such as Count pages etc, will have delays, but most of them run in the background which means you can do other things if they're long running. But those delays have nothing to do with the size of the library.
My reason for several libraries is that I want different custom columns for different content - books, audio/video, papers - and there's not a lot of commonality between them.
BR