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 consider 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.
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 the 'Beginner's Mind' is a state of mind that can still be achieved in the presence of knowledge, but nevertheless, one can see how it might be easier to achieve in the true absence of knowledge.