Computation within sl_customization function

I currently use sl_customization to make my own menu in the menu bar; it has several menu items and some more sub-menus.

I'm trying to generate a new sub-menu which lists a number of blocks within a Simulink design - on large designs this number of blocks can exceed 1000, which makes MATLAB hang. I've added a cap of 50 on the number of things added to the menu which stops the hanging, but the computation of the list of blocks seems to have to be done within sl_customization and is making MATLAB freeze for a few seconds every time the user clicks on the menu in the menu bar.

Is there any way I can get MATLAB to do this computation in a different thread or something? Or at least cache it somehow?