Well just offhand, you could recode slightly as
a = [g(i:i+9), j];
We cannot do much more without knowing whether xyz is a function or an array. If it is an array, then you are attempting to use 10 indices in the first dimension, and the result is not going to fit into the single location p(j). If it is a function, then all essential speed-up is going to depend upon what the function does and how it can be re-coded to work on vectors.
I get the impression that your xyz is a function that is doing some kind of filtering or convolution ? convolution can be coded more efficiently using conv() or filter()
