There's some "blocking" anyway currently if I understood the OP correctly - simply in the form of the delay until the form is shown (which is irritating to the user).
If loading in a parallel thread upon opening the form, the initial state of the form would have to be something like Enabled = false.
So yes, the form UI needs to represent that it's not ready yet, but even that is way more friendly to the user, who then knows something loads (maybe even sees a progress bar if it's really heavy stuff), compared to clicking to open a form and nothing happens for a while.
I wouldn't call it "the form would have to wait", but "the form needs to change its state upon thread termination".