Is activating filevault will slow my computer, my web surfing, my processor or anything bad i ignore please ?
It's meant to encrypt your home folder mainly in order to protect what sensitive data you may have there (hacking, data stealing malware, backdoor or Trojan, or from anyone with physical access to your computer.) It has gotten better in recent OS versions--Mountain Lion--but I wouldn't touch it in anything earlier. It's known for creating huge problems. It deservedly got the nickname "vilefault."
Much better to put your sensitive data, whatever it may be, in a password protected, encrypted disk image. Be sure never to lose the password and keep a backup copy or two in case one gets corrupted. Be sure not to allow Keychain to remember the password, or that defeats its whole purpose.
And make sure you have a backup/clone of your entire Mac on an external hard drive. You can use Carbon Copy Cloner, my preferred, or SuperDuper for this.