I’m developing an Android application but it takes some time to load on the phone and it’s not that fast when it’s running. Can you give me some advices about how make it lighter and faster? I have quite a lot of textures, but they are all in .png format, which i think is the best compression method and supports trasparency too. What settings (textures and everything else) should i choose? Should i convert the textures in a different format? What in the code (in general) is expensive on the phones? Thanks in advance! 
PNG is a file format used to import textures into Unity. They should be converted into ETC textures for Android. See:
http://docs.unity3d.com/Documentation/Manual/android-GettingStarted.html
Typically games on mobiles are slowed down by the speed of their graphics systems and not the time it take to execute script code.