/performance

name: unity-performance
description: Advise on Unity performance red flags. 为 Unity 性能红线提供建议。

Triggers

• Reviewing performance
• Diagnosing frame drops
• Reducing allocations
• Planning pooling/optimization
• 做性能审查、诊断掉帧或卡顿、减少内存分配、规划对象池/优化

Unity Performance Red Flags

Use this skill for a high-signal review of likely Unity performance issues. Focus on red flags, not speculative micro-optimizations.

Check For

• Too many unrelated `Update` / `LateUpdate` / `FixedUpdate` loops
• Repeated `Find`, `GetComponent`, `Camera.main`, or tag lookups in hot paths
• Frequent `Instantiate` / `Destroy` suitable for pooling
• Avoidable per-frame allocations:
• LINQ
• string formatting
• closures
• boxing
• Reflection in runtime hot paths
• Expensive editor-only helpers leaking into runtime code
• Physics, animation, or UI updates happening at the wrong cadence

Hidden Costs: Possibility ≠ Actuality

Three counter-intuitive traps where what seems "free" is actually expensive. Each cost is paid for *possibility* of work, not for work actually performed — which is why profilers rarely catch them directly.

Write permission costs, even without writing

A component whose `Update` *can* mutate `transform.position` pays the cost whether the branch runs or not: dirty-flag and serialization systems treat write permission as a reason to poll every frame. Similarly, a `[SerializeField]` field that is never modified at runtime still sits on the serialization path. The fix is to **remove the permission**, not tighten the branch — split the rarely-needed writer into its own component and `AddComponent` only when needed. *Source: `NetcodeSamples/HelloNetcode/2_Intermediate/07_Optimization/Optimization.md` — "a system with the possibility of writing to a component, regardless of whether it writes to it or not, will always have to be serialized".*

Sequential seeds produce correlated random streams

Seeding N RNG instances with `baseSeed + i` gives N *similar* streams — patrol paths line up, spawn jitter clumps, loot rolls cluster. Hash the index before seeding:

// Wrong — N correlated streams
for (int i = 0; i < N; i++) rngs[i] = new System.Random(baseSeed + i);

// Correct — hash decorrelates adjacent seeds
for (int i = 0; i < N; i++)
    rngs[i] = new System.Random((int)((uint)baseSeed * 2654435761u ^ (uint)i));

`Unity.Mathematics.Random.CreateFromIndex(i)` applies this internally and is preferred when the math package is available. *Source: `Dots101/Entities101/Assets/HelloCube/3. Prefabs/SpawnSystem.cs:37-39` and its comment.*

Logging fires in release builds by default

`Debug.Log` is **not** stripped in Player builds; only methods marked `[Conditional("UNITY_EDITOR")]` (such as `Debug.DrawLine`) have their arguments elided at the call site. That means a log line with interpolation or helper calls runs every frame in shipped games:

// Wrong — GetPlayerInfo() and string interpolation execute in release
Debug.Log($"Player {GetPlayerInfo()} at {Time.time}");

// Better — guard the whole expression
if (Debug.isDebugBuild) Debug.Log($"Player {GetPlayerInfo()} at {Time.time}");

The same principle as the "possibility write" rule above — the runtime pays for the *possibility* of work, not only for the work.

Output Format

• Confirmed red flags
• Likely red flags
• Changes worth doing now
• Changes not worth doing now
• Expected gain category: clarity / frame time / GC / scalability

Guardrails

> **Mode**: Documentation only — no REST skills to gate; load freely under any operating mode (Approval / Auto / Bypass).

• Do not recommend large refactors without a meaningful hotspot.
• Do not replace simple code with unreadable “optimized” code unless the hot path is real.