X Xbox One Frame-Rate (FPS)

Recently held DMC5,the game was just fine (because of iron ONE X),almost no problems,but having thrown the game for a few days and returning,I noticed unpleasant changes.FPS started to drop,and significantly,in those locations where it was not before,and well if if if a strong mixing,so there is sometimes the opposite.Fighting with the crowd of demons and all the rules,and can you run through the location and FPS greatly drops and Vice versa.Again,when just started the game and went to half,it was not,and I played in 4K,then cut down due to the fact that the TV is not very large but staff wanted more and still been fine without a problem.Tell me what could be the problem?I wanted to check on other games,but I mostly either shooters and Forza,games that on a normal box going well.So I can't understand what was going on,watched the video on YouTube frame-rate,so there is not even a hint of sagging,where I have,it started about three chapters before the end,so I suffered a drawdown(in the last Chapter they are almost not there)and walked more into the game did not go.The box works as usual,no noise,dust clean.Maybe the TV?But again, before I did not notice.