Short version

Lag scales with how many projectiles exist and how many targets each one has to check every frame. Towers that fire many small projectiles, pierce through crowds, or constantly re-target are expensive. Towers that deal huge damage in few hits are cheap. The community's lag-heavy and low-lag freeplay setups line up with that rule exactly.

The rule

Each frame, the game moves every projectile and checks it against the bloons it could hit. The cost is roughly projectiles × nearby bloons. So anything that multiplies projectiles (fast attack speed, multi-shot, splash that spawns sub-projectiles, long-lived piles) or multiplies targets (dense stacks, high pierce) multiplies the work. A tower that fires once a second and deletes a B.A.D. costs almost nothing by comparison.

Setups reported as lag-heavy

  • Bloonjitsus with Shinobi Tactics stacks. The Bloons Wiki's freeplay strategy page says plainly that this setup "produces immense lag" (source). Dozens of fast-firing ninjas, each shuriken checking against a dense stack, is the worst case for the rule above.
  • Spike Factories. Spike piles persist on the track, and every pile is a projectile the game keeps checking. Players on the Steam forums name spike-based defenses as a common lag source.
  • Perma-Brew and other buff-spam. Buffs that apply to many towers add per-tower bookkeeping every frame.
  • Target-hungry upgrades. Steam users report that removing 2-x-x Submarines, x-x-3 Monkey Aces and x-x-1 Ninjas gave them frame rate back; those upgrades spend a lot of CPU finding targets (thread).
  • Adora in hybrid Temple + Ninja setups is noted by the wiki as adding lag compared with Ezili in the same role.

Setups reported as lower-lag

  • Vengeful True Sun God with Sun Temples and Sun Avatars. The wiki describes this as "somewhat weaker than Bloonjitsus + Shinobis, but it produces less lag and is easier to achieve." Few towers, enormous per-hit damage, bloons deleted at the front of the track.
  • Kill at the front. A Steam user's advice: fewer towers that attack slower but hit harder, and kill bloons at the front so stacks never build up behind them. Dense stacks are what make every projectile expensive.
  • Paragons over spam. One Paragon replaces a field of tier-5s. The projectile count per second drops even if the damage goes up.

Practical checklist for a long freeplay run

  1. Before round 200, sell towers that aren't contributing. Every idle tower still runs target selection.
  2. Avoid stacking fast-firing, low-damage towers. If you need a stall, use one strong stall (Glue Storm, a single Icicle Impale) rather than many.
  3. Prefer towers that kill at the entrance so bloons never accumulate.
  4. If frame rate drops sharply after placing one tower, that tower is the problem. Players report individual placements in late waves causing immediate drops.
  5. Leave headroom if you record: the recorder needs some CPU too. See does BTD6 use CPU or GPU?
Note on sources

Ninja Kiwi doesn't publish per-tower performance data. The lists above come from the Bloons Wiki and long-running Steam forum threads, plus the projectile-count rule that explains them. Treat specific tower claims as community-reported.

Already recorded a laggy run?

The lag is in the file as repeated frames. LagCut removes them and exports the real gameplay at one speed; see how to speed up laggy BTD6 footage.