Reproduction steps:
1. Open the attached "Load" project.
2. Build and run the "MainScene" for WebGL.
3. Click space.
4. Notice the printed time after a few seconds (It shows loading time).
5. Compare it between different platforms and browsers.
- Editor - 0.33 s
- OSX Standalone - 0.25 s
- WebGL Firefox - 1.33 s
- WebGL Chrome - 2.94 s
- WebGL Safari - 2.48 s

Notes:
- May be related to _UNITY_LZ4_decompress_safe function, that was called from Resources.Load.
- Prefab size should be ~2+ Mb size.

Workaround:
- Load files through Asset Bundles.

Reproduced with:
5.5.0b1, 5.6.0f3, 2017.1.0a6