Regarding to the performances, should I check if a post has a thumbnail before or after checking if is the front page (or single page, etc.)?

Neither, they are just as fast as each other, both are using information that has already been fetched in advance. This is micro-optimising. Functions such as is_front_page or is_singular etc etc are just looking up variables already set in the main query object. Likewise functions such as has_post_thumbnail etc are looking up data that has … Read more