What is better: to have a modal open instantly and then load its contents, or to load its contents and then open it?

Let's say you have a button. When you click it a GET request is fired for some data (for this example assume that the data cannot be prefetched). Then a modal opens and displays that data in a pretty way.

For user experience, is it better to immediately open the modal but have the contents be blank until the data is loaded, upon which the data is painted

or...

Is it better to wait for the data to come back and then immediately open the modal with all of its data already there.

In the former case the button click is more responsive but there is a "flash" from white to contents.

In the latter case the button click isn't as responsive but there is no flash and reflow of contents. Instead, everything is just there already.

The GET request usually only takes 10-30 milliseconds if that changes anything.