Why does my phone suddenly drop in service when I play my favorite game?

Hello. I play a game called vainglory, its I'd say a large game with alot of things going on at once. Its a MOBA.
I play this game on my galaxy note 5 (which it brand new) with everything up to date. Now it seems I have excellent lte service before I go to play, but once I open the game app itself everytime my service drops to a very low 3g signal and im so unclear on why? It makes me lag and causes very late response times and it cause my teammates to get mad. If anyone can shine some insight on this id really appreciate it. My service provider is sprint and ive contact them but they where little to no help.
Any information or feedback would be awesome
Thank you.