I can certainly understand your point of view and dismay at your phone. If my N4 lagged this much, I would be posting vicious comments here.
Many problems can be caused by other apps... even if they're disabled and not running. Even constant syncing can cause problems... especially if there's some problem with the files being synced. Also, since this whopping delay concerns the gallery, I would check to make sure your SD card is not at fault (if you're saving photos to your SD card). Try swapping the card, reseating it, or removing it entirely.
As a last recommendation, unfortunately, you really need to factory reset the phone, don't install any apps, and see if the lag is still present. If it is, you should talk to whoever you bought the phone from and see if you can get a replacement. At the very least, go down to see them so they can have a look at it.
Good luck!