Hi -nth-,
Just out of curiosity, how large is your XML data file? When dealing with XML files, the larger the file, the slower it gets on the design interface, regardless of any presstalk. You can even start experiencing slight slow downs starting at 2Mb XML file. If dealing with files past 10Mb in size, the performance will dramatically be affected, even to the point where the computer stops responding and crashes.
Before starting to code many functions, try to see if dealing with small samples of data helps. In fact, no matter what data type is used in the form, we always recommend to design with small files containing a few records, when possible of course.
Hope this helps. Regards,
Olivier