If your iphone is going slow then you might be able to speed it up significantly by deleting your photos and videos.

the first thing to do is to make sure you have all the photos synced to your pc or mac.
once you are sure they are all on the computer you can then

• go into the photos app
• click the album name
• click the icon in the top right
• select multiple photos
• click delete

once you have them all deleted your phone should be much faster

you should also disable itunes photo sync from putting photos on your iphone.