If I'm not mistaken, all data operations done on references must be done in the UI thread. Depending on what thread the sub-vi is normally running in, this can have severe performance issues.
Have a look <a href="http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=135&HOID=50650000000800000026CB0000&USEARCHCONTEXT_CATEGORY_0=_49_%24_6_&USEARCHCONTEXT_CATEGORY_S=0&UCATEGORY_0=_49_%24_6_&UCATEGORY_S=0";>here</a> for more information. Shane.
