[racket] gc and vectors

I have a simple program using vectors of inexact numbers.
Half the time goes to garbage collection.
I suppose that's because vectors effectively are internally arrays of
pointers to the numbers.
So reassigning an element to a vector may leave garbage.
Do unsafe floating point vectors and operations use arrays without
intermediate pointers?
If that is the case I could speed up my program by a factor of 2.
But before trying that I would appreciate advice.
Thanks, Jos
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