Should code be rewritten or should I use methods?

First of all, I couldn't find this question, so I decided to post here.

While working on a game, I realized the more entities I have on the playing field, the more the fps drops, which is expected.

To optimize it, I read somewhere (I'll post the link if I can find it) that if you rewrite code instead of using universal methods, it is faster, because it doesn't have to 'link' the methods each time it runs the code. But, since I am rewriting code, It might take up some more virtual space.

I just want to know your opinions on this, and if you think it is true. And I would gladly accept any other optimization tactics or links.

Edit: What's the point of making the question 'too broad' after the answer is selected? If the answer was selected it means there was a valid answer, which means that the question was not too broad, otherwise the answerneers would have had problems...

Thanks