The problems are not only related with the graphics, the code is not precisely my best work and have a several bottle necks. Even the game design itself sucks (the enemy "AI" for example).
Remember that this game is just only a small practical excercise (the first of 5) for a Unity Game Development Master Degree that I taken in 2017, made it in just only 4 weeks, with several technical restrictions imposed by the teachers (precisely to learn how not to do some things in Unity). A full menu with settings to setup graphics or input bindings was not a requeriment for the practical excercise (or the optimization in general terms).