Based on the comments, the problem is 100% not how quick the Druid's shapeshifting is, it's that you haven't been making GM moves on 6−.
Restricting the move is not at all the answer. The GM doesn't have that authority in the first place, and more importantly, the Shapeshifter move needs GM moves on its misses in order for it to function correctly. If you haven't been making GM moves on a miss, the move is going to be completely unhinged.
Don't change anything else yet — fix the lack of GM moves first, and see how the move is supposed to work. You'll notice that it's still quite powerful, but so are the downsides when things go wrong. Play at least a session with the rules as written before attempting to hack it to fix a problem that the move only has because of the lack of misses.
As an important tip: on a 6−, the GM always makes a move, regardless of what the player move says. (And the player always gets to mark XP — that is another thing that isn't under the GM's authority.)
But since we're here… how long does it take the Druid to shapeshift?
Like many unwritten things about Dungeon World characters, the easiest way to find out is to ask the expert: the Druid herself!
GM: Druid, how long does it take you to shapeshift?
Druid: It's the blink of an eye! One moment I'm there, then a blur and I'm a bear. If you blink, you'll miss it.
GM: Cool, okay! *makes a note*
GM (writing): Druid shapeshift: blink of eye / blur. GM move idea: so fast that when something goes wrong with a shift, by the time Druid notices, it's too late.
This is a pair of GM Principles in action: Ask questions and use the answers and Be a fan of the characters. The Druid is the expert here, and that's great — let her player tell you how the Druid works, when Dungeon World doesn't already say. Then build on her contribution to fill her life with adventure, later, by using those answers for good and ill. No matter how awesome-seeming a player's answer about their character's abilities is, there are ways it can be good and bad, and the GM can build on it to make the PC and game more interesting in the future.
There's also a sneak peek at a possible future GM move (Show a downside to their class, race, or equipment) at the end there. The nice thing about Dungeon World is that, because you have full permission to make things go beautifully, entertainingly wrong on a miss, you can relax and let yourself let a move go beautifully, entertainingly right when it's a hit. Dungeon World characters are powerful, and the game wants to see them use it to change the world. It's normal and okay for the PCs to enjoy huge successes without the GM tempering them, because sooner or later something will inevitably go wrong too.
My own experience asking my Druid questions about how her class works can be illustrated with a brief tale:
Two DW campaigns back I had a Druid in the group. We never learned exactly how fast or quick the shapeshifting was, but when I asked how it worked, we learned that it required a physical totem with the relevant nature spirit inside, for each shape the Druid could take. Shifting involved asking that spirit to lend Druid its form, and having it hold onto Druid's human form for safekeeping. That Druid was a combat monster, wrecking organised foes with Elephant charges and swooping around with tactical rakes as Eagle.
But building on this answer lead, with no planning at all and only in hindsight, to the climax of the campaign where the party vied with a drow priestess to see who could raise a greater spirit to demigod-hood first, and hence who would influence its alignment and attitudes, and its future as a god in the Forgotten Realms.
All because of one missed Shapeshifter roll. The miss let me build on the Druid's own answer on how shifting worked, and combine it with a random dark pit that I'd added to the current dungeon room without ever knowing what it was going to be. When the Druid missed on a Shapeshifter roll while about to fall into the pit as Elephant (the intent was to become Eagle), the obvious “make a move that follows” result that built on everything I knew about the situation just happened to be that Druid was trapped in Elephant's form, because the pit was a Gate of Elemental Darkness and something down there sucked Elephant Spirit out of the totem, so that Elephant couldn't trade back forms with Druid. From there, history was made…