Reserving/Preallocating associative array?

[Gordon]

Hello,

I want to load a large text file containing two numeric fields 
into an associative array.
The file looks like:
    1   40
    4   2
    42  11
    ...

And has 11M lines.

My code looks like this:
===
void main()
{
         size_t[size_t] unions;
         auto f = File("input.txt");
         foreach ( line ; f.byLine() ) {
                 auto fields = line.split();
                 size_t i = to!size_t(fields[0]);
                 size_t j = to!size_t(fields[1]);
                 unions[i] = j; // <-- here be question
         }
}
===

This is just a test code to illustrate my question (though 
general comments are welcomed - I'm new to D).

Commenting out the highlighted line (not populating the hash), 
the program completes in 25 seconds.
Compiling with the highlighted line, the program takes ~3.5 
minutes.

Is there a way to speed the loading? perhaps reserving memory in 
the hash before populating it? Or another trick?

Many thanks,
  -gordon