Writing less and keeping a good performance, is it possible?

These past few days I've been thinking of a way to avoid needing to write a lot of code and still keeping a good performance for a Air desktop game I'm developing, as a hobby.

The game is a sort of vertical shooter, that consist of several entities moving and checking collision. There are plenty of different kind of units. Each frame I have something like:

entity.execute();

The simpler approach is to have all different entities to inherit the Entity class, and manually customize them all. This is slow and cumbersome, and hard to maintain. But it's fast, performance wise.

The other approach is to have only one Entity class, and just using some sort of composition to simply add "behaviors". So for example I have a master class with things like types of movements, attacks, etc, and the different entities use them.

The problem with that approach is, calling a function is slow, according to my tests, it is ~3 times slower than just having the code right there (inside execute()).

I'm in a dilemma, I can't find a way to reuse chunks of code to decorate generic Entity instances, and keep a good performance. Seems like I have to use one or the other.

I tried using [Inline], but I've read it's not a stable feature, and I didn't see any noticeable performance improvement, I didn't test it much though.

Any insight is appreciated.