If debugging a JavaFx app is unacceptably slow, disabling component tracking will help :
Tools -> Options -> Java -> Java Debugger -> Visual Debugging : uncheck "Track locations of component hierarchy changes"
This option may be usefull when one need to know when and where a component was added to the hierarchy, and useless the rest of the time.
It’s filed as bug 241082 in Netbeans, and turning the option off is the most sensible thing to do.
As discussed on : http://mail.openjdk.java.net/pipermail/openjfx-dev/2014-January/012533.html