[chaley]

11 Mar 2021 (in calibre source):

• Performance improvement of the virtual_libraries() template function
  
  The performance of the virtual_libraries function has been improved substantially. For example, on my Windows machine using a test database with 3900 books, 15 virtual libraries, and one composite column that calls virtual_libraries(), the performance change is:
  

  First call to virtual_libraries()
               old = 368,381 μs
               new = 382,352 μs
  
  Typical time for each call thereafter (once per displayed book), until data changes:
               old = 165,338 μs
               new =       1 μs

  Result: scrolling 20 books goes from 3.3 seconds to 20 microseconds.
  
  On a smaller library of 200 books with 5 virtual libraries the numbers are:
  

  First call to virtual_libraries()
               old = 7,852 μs
               new = 7,846  μs
  
  Typical time for each call thereafter, until data changes:
               old = 195 μs
               new =  <1 μs

  The performance improvement is directly related to the number of virtual libraries.