Hello everyone,
I just downloaded the dragonborn DLC on my xbox and played it without any issues.
But after a while, my game begins to lag all the time!! its crazy! i hope u guys know something about this.

If you haven't tried this already, follow the instructions on the following page (under clearing the system cache): http://www.gamesas.com/topic/1426775-starting-dragonborn/page__view__findpost__p__21895466
This helped my game to run more smoothly, so maybe it will help you out.

I did delete the cache but it still lags a lot.. I also downloaded the dlc again. but nothing worked. i heard something about the museum key glitch.
but that was also not the thing. I really need help with this

Another possibility is that there may be a lot of weapons and shields lying around at enemy spawn points in the game. If you're not aware of this, there is a bug where weapons and shields don't clean up with the bodies of slain enemies. It can help reduce the amount of lag in the game if you pick these up and put them back in the slain enemies' corpse after you kill them. After a period of time, the number of weapons and shields leftover can really start to build up causing lag, especially in areas that are visited frequently.