I've been using the solution here
http://yabb.jriver.com/interact/index.php?topic=73415.msg497686#msg497686 to access pdf booklets.
Recently (possibly since Version 20) I've noticed that the startup time for MC has slowed down to about a minute. Checking the log, it seems that the delay is due to the Filter [=IsMissing([Filename (path)]Booklet.pdf)]=[0]</Item>. By the sound of the churning of HDDs, this filtering is done at startup rather than the check being done at the time of accessing the link.
Removing the filter removes the startup delay but creates a booklet link whether or not it exists.
I wonder if anyone has any ideas for a work-round or an alternative way of displaying pdf links when they exist.