Any advice on using matlab coder efficiently?

I am trying to learn how to use matlab coder to generate mex functions in order to speed up my codes.

I read the documentation (at least parts of it and watched tutorials) so I feel I have a basic understand how the matlab coder works, but my first experience has not been encouraging (the mex code was slower than function despite a massive for-loop that I compute withing the functions). So I wonder if there is a way to write m-files that are to be translated to mex files in order to improve efficiency of mex functions? More specifically, here are my questions.

1. Are there any don'ts for preparing m-files (in terms of coding style) that are to be converted to mex-files? For example, I read that logical indexing is slow in mex files.
2. Is it OK passing structures to the function that is to be converted to a mex file. I have many parameters that I would like to pass and having passed them in a structure makes the code much cleaner.
3. In m-file having a+B where a is a scalar and B is a matrix is fast. Is the same true for mex files or should I transform a into a constant matrix?

Any other advise would be much appreciated.