I believe that QlikView is using IEEE double precision binary floating point internally. That creates some problems, but the storage of 255 decimal places is not one of them.
I have no idea why it's displaying all those decimal places or why the dummy column fixes it. But generally speaking, I let QlikView do whatever it wants as far as numeric storage, and I simply format the result as desired. To do this in the document as a whole, you can use settings -> document properties -> number. For specific charts, it would be properties -> number.
Definitely don't do a round() during the QVD load or add a dummy column. As you've observed, this can kill performance if it takes you from an optimized QVD load to an unoptimized QVD load.