I also have several custom columns, because tags aren't always the most convenient way to display something. Still, wherever possible, I try to use tags and/or saved searches instead of custom columns.
I do have a separate library for the books I load on my ereaders and intend to read in my remaining lifetime. Nevertheless I mostly work with my main library which has all the ebooks I have. It takes several seconds to open on my latest PC with 16 GB RAM and an I7 8th gen. processor. I have about 10 custom columns there.
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