Songs playing is lagging with locked screen and wi fi

[IBen94]

Hey guys, 

I have a problem with playing songs with locked screen and wi fi on.

Songs after few seconds starts interrupting every 3-4 seconds but only if the screen is locked (With 3-4g playing is ok) . If I use the phone while I'm listening Spotify the reproduction is ok. I have a Samsung S6 edge and Spotify premium  is updated. I have this problem since some months and it's very annoying.