I experience the same problem, which especially appears when storage use is growing towards 1GB. It does not depend upon the resources of the computer (and I have a big one 
). What I never do is use definition of variables with immediate output of the value in the definition formula. Their update behaviour is much worse, so the functionality is essentially crap.
Unfortunately, there isn't any such file read/write. I agree that it would be very useful sometimes if there were.
Writing such a file would actually be possible. The data values are of course easily available. In Mathcad everything is stored as a double precision float, so we don't need a variable type, except to define which numbers belong to a particular array. That would be easy enough. Quantities with units are always stored internally in SI. Getting the units is a bit more tricky, but possible. A scripted component could be used to get the region XML via the automation interface (I have such a component already, in fact), and that contains the unit information.
Reading the data back, and creating an appropriate variable, is also possible. Assigning values to a variable is easy, but there is no easy way to assign units (this was easy in version 11, and will be easy in Prime, but in versions 12 through 15 it is not). It can be done by modifying the XML of a region that already exists though, again via a scripted component (and again, I already have such a component).
It would be a lot of work though, and it would require the use of scripted components to handle the unit information. It would be much better if PTC provided such functions (Hi Jakov, are you reading this? 
).
