Adoby
I recently had start times go from a few seconds to a few minutes if I started on a specific library, and switching to the library also became slow. It started when the library reached about 6,000 books. The library also became sluggish when scrolling the library list etc.
However, I have another library with 32,000 books, starting or switching to it takes just a few seconds. So it was obviously something specific to that 6K book library.
One of the custom columns on the 6K book library was derived via a union of four other columns, each of which contained a list of names. When I removed that derived column the start/switch times went back to a few seconds. After rearranging the order of columns in the library list, and in Book Details I was able to arrive at a layout where I didn't lose much by not having the derived column.
It was interesting that the presence of the derived column had no apparent effect until the library went through a tipping point. I played around and found that the start times started to rise at about 5600 books and became progressively longer so that at 6100 books the start time was about 5-6 minutes
I suspect the tipping point is a function of the other factors mentioned by Adoby as well as the nature of the derived column that creates the delays. That library has two other derived columns that use simpler templates, it also has several column icons - removing them makes no appreciable difference to start/switch times but they probably have there own tipping points.
BR