Great question — this is a subtle but important point in GPU rendering performance. Let me break it down in the context of your image and the phrase:

**“zig-zag topology increases the chance for quad leakage, which is multiplied by MSAA.”**

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### 1. What is **quad leakage**?

Modern GPUs shade in **2×2 pixel quads** (called “fragments”) rather than on an exact per-pixel basis. This allows for things like derivative calculations (`ddx`, `ddy` in HLSL/GLSL) and efficient SIMD execution.

* If a triangle only partially covers a 2×2 pixel quad, **all 4 pixels in the quad may still be shaded**, even if only 1 or 2 are visible.
* This extra shading work is called **quad overdraw** or **quad leakage**.

So, even if you render 421K pixels (as the image says), the GPU might actually execute shading on significantly more fragments.

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### 2. Why does **zig-zag topology** make it worse?

Looking at your wireframe:

* You can see lots of **thin, diagonal, zig-zagging triangles** instead of clean, rectangular or strip-like geometry.
* When geometry cuts across screen space in odd shapes (especially diagonals), it tends to intersect more 2×2 pixel quads inefficiently.
* This means more quads are “leaking” shading work — i.e., more fragments get shaded that don’t actually contribute to the final image.

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### 3. Why is it **multiplied by MSAA**?

MSAA (multisample anti-aliasing) increases the number of **coverage samples per pixel**.

* For example, 4× MSAA means **4 coverage tests per pixel**.
* If quad leakage causes the GPU to shade “extra” pixels, then under MSAA, each of those extra pixels must be evaluated across multiple samples.
* This multiplies the wasted work: **quad leakage × MSAA sample count**.

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### 4. Why is it relevant here?

The image shows:

* Large textures (4K×4K, 2K×2K).
* A relatively modest pixel count (421K).
* A wireframe with zig-zag patterns highlighted.

Despite “only” 421K visible pixels, the zig-zag topology caused high quad leakage, and with MSAA enabled, that ballooned the shading cost.
That’s why this particular draw call took much longer than average: **too much wasted fragment shading due to poor triangle layout + MSAA overhead**.

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✅ **In short:**
“Zig-zag topology” means triangles are poorly aligned to screen-space quads. This increases **quad leakage** (shading fragments that aren’t actually used). With MSAA, each of those wasted fragments requires multiple evaluations, so the cost grows dramatically.

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Do you want me to illustrate this with a small **2×2 quad diagram** showing how a zig-zag triangle can trigger extra shading compared to a well-aligned triangle?