They are both in either’s powers; but this swift business
I must uneasy make, lest too light winning
Make the prize light.
– William Shakespeare
The Tempest, Act 1, Scene 2. Prospero has learned not to trust appearances, especially after how he was treated by the brother he trusted. So while Ferdinand appears to love Miranda and they are attracted to each other, Prospero is determined to put the strength of that love to the test. By "this swift business" he means their love at first sight.