Retrieving data from a database in the background while a dialog is loading

I'm using Dapper in a WPF application to retrieve data from a database for a couple of combobox lists. I'd like that retrieval to occur in the background; it will prevent a small delay when the user opens the first combo box.

So I've done this:

private Task<IEnumerable<T_Program>> _allTapes;

// Binds to combobox ItemsSource
public IEnumerable<T_Program> Tapes => 
   _allTapes.Result.Where(x => x.Program.Equals(Program));

And in the constructor of my View Model:

_allTapes = _conn.GetAllAsync<T_Program>();

But I didn't get the "performance improvement" that I wanted.

Hovering the cursor over _allTapes during debugging yields the following description:

Id = 6722, Status = WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"

So it would appear that Dapper's GetAllAsync method doesn't actually execute the query until it is compelled by retrieving Result from the Task.

How do I get the background execution that I want?