Yep, playing your best in COD and leveling up fast in COD are basically incompatible. To play well, you have to be fresh and sharp. To level up fast, you have to keep playing long past the point where you get groggy and start slowing down. More than a few times I forced myself to keep playing until 3am even when I was logging 5 kill / 15 death games, because I knew it would bring me closer to weapons like the PTRS sniper rifle.