I absolutely agree, I advised my father not to upgrade and Apple should know this too.
In my mind they have unscrupulously destroyed the phone in order to force people to buy a new handset. The i4 zipped along on iOS5 and was workable on iOS6, despite being slowed considerably even by that upgrade. I would implore anyone who hasn't already upgraded their i4 not to do so. They will suffer a very noticeable slowing of all functions and also worse battery life. Why bother? The new features certainly aren't worth killing your handset for.