With mortars, you may want to start practicing narrow tunnel construction. All other ants are most efficient when spammed at lvl 1 only, no upgrades (mortars aren't exempt either if you don't make use of their AoE, they'll do more damage to single targets at lvl 1). The only upgrade worth getting is level 3 mortar, as that's the level at which they receive AoE. You might be missing a critical part of the game's current META. Mortars are also more powerful all-around given that your computer runs slowly (lag has a greater impact on rapid-fire since they are required to get off more shots to do their usual damage, allowing mortars to consistently get a higher benchmark it sounds as if your PC doesn't have this problem). You haven't played 2.2 insane, have you? Lvl 3 mortars are a legitimate requirement to beat that level, as it is so overwhelmingly difficult.