Speeding up the creation of Instance from a several megabyte string

I'm creating a somewhat large model and creating a `Model` via the `add_string` method. Then, quite a bit of time is spent on creating an instance of `Instance`. My Minizinc instance when written on the disk takes about 8-9 megabytes. 

The docs don't seem to touch upon the issue, but are there some general guidelines or best practices to apply when handling instances of this size (or larger)?