In dot.m we find a test for integer types, checks by isvector, comparison of lengths, both vectors are shaped by (:) and after a test with isreal(), either a'*b or sum(conj(a).*b) is called.
means, that the tests are expensive and the two (:) operations also, because they create deep data copies in modern Matlab versions for complex data:
a = rand(1,100) + 1i * rand(1,100);
s = formattedDisplayText(x);
c = strtrim(strsplit(s, '\n'));
With real values only, the data are shared. (For other readers. I'm aware, that you, Bruno, know this.)
My conclusion: dot.m is not efficiently implemented. Why? For future improvements.
I do not see a reason, why (:) creates a deep data copy, so I'd consider this as a bug. Did you ask MathWorks already for an explanation?
By the way, Matlab's cross() could be 10 times faster when implemented as MEX function.