Thanks for your feedback. I've started defragmenting (it has turned out that the main db file was quite heavily fragmented, and quite many .opf files are fragmented, too) and we'll see (for now, I've closed Calibre).
One question about rules: is there any method to disable/enable them instead of removing them and setting up once again?
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@Chaley, as the 17 rules are used to display only one type of icons in one column, I guess it has to be possible to replace them by an advanced/composed rule. Thanks for the tip! Now I just have to understand how advanced icon rules/custom templates work
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@DaltonST, that's basically what I do (with views and VLs) - a custom view for most of VLs, and irrelevant columns are hidden in other VLs.
Thanks for the tip about defragmenting!