Reducing the number of custom columns will definitely have *some* performance impact, although how much in the grand scheme of things

???
P.S. Change:
Code:
cache = init_cache(library_path = '/Users/jasper/Documents/Calibre Test Library/')
to
Code:
from calibre.utils.config import prefs
cache = init_cache(library_path = prefs['library_path'])
That way you use the default library path, instead of requiring users to modify the script.