Learn how to force objectives earlier.
If your enemy team sucks for 40min and you're losing a 5v5 then the game, you're doing something wrong.
First, if they suck for 40 min that's more than enough time to get 5 dragons. Pretty much ggwp?
The key point is IT'S EASIER TO FORCE DRAGON THAN A TURRET. An enemy team may be able to stall out by defending turrets until they outscale you, but if you're ahead they shouldn't be able to stop you taking each dragon as it spawns at will. To an extent you should also be able to force Baron. Grab a pink and a sweeper, make sure they have no vision of Baron so they may think you're taking it and sweeper the river bush so they don't know you're in it. Either they take the bait and you destroy the first people to go to river bush, or they show up on the map and you take a free Baron.
Kills are not the purpose of the game. Kills should ALWAYS lead to objectives. If you have a lead, use those kills to force objectives. If you can't force turrets, force dragons and barons. Once you get the buffs, forcing turrets should be easy.