Performances with many fields per content type Drupal creates a new table for every new field in a content type.

How much this affects the performance in fetching data when the fields of an entity are 15 or more? 

Is possible to store all fields in a table (with a new entity?) without drawbacks? Is worthwhile?
MySQL limits you to 61 table joins but Drupal doesn't care about this limit. Each field gets loaded individually, so every field will slow down Drupal a little bit but there is no practical limit. The impact depends on how long a round trip to the database takes. If you have Apache &amp; MySQL on the same box and you're using sockets for communication then the impact will be minimal (read slides 60-68 from my presentation for more info). If Apache &amp; MySQL are on different servers then the number of fields will have more of an effect. All of this can be mitigated by using something like the https://www.drupal.org/project/entitycache module.