Is there anything about learning music theory well that slows down your musical capabilities?
Definitions and sets
Partly, it depends on your definition of 'music theory' - whether, according to one's definition, the term 'music theory' covers all the knowledge one can have about making music, or if you consider that only some of the knowledge you can have about music comes under the label 'music theory'.
If you are of the opinion that 'music theory' is only some of the set of musical knowledge, then clearly it's possible that, for a given musician with a given set of goals, they'd be helped towards those goals more quickly by leaning more of the non-'music theory' knowledge than by learning the music theory knowledge. In that sense, if you consider the opportunity cost of learning the music theory, it's clear that learning it would slow you down.
Of course if you consider 'music theory' to be any knowledge you can have about music, that wouldn't make sense - it depends on your definition.
The value of the Beginner's Mind
It's not necessarily true that more knowledge makes you better at something - the idea of the 'Beginner's Mind' is often valued when it comes to creativity. Of course some might say that this is simply a state of mind, and one that can still be achieved in the presence of knowledge, but nevertheless, one can see how it might be easier to achieve with less knowledge.
Introductory courses often don't give you the bigger picture, and may even obscure it to an extent by not telling 'the whole truth'.
To quote Dom -
People take or see an intro music theory where your hand is being held to learn and compose a certiant style and it scares most people away without them seeing a bigger picture.
But that invites the question - why are so many into music theory courses not good at leading people towards the bigger picture?
I honestly think that a lot of beginners music theory material is lacking in context and often has a - perhaps unintentional - presumption towards certain styles.