D3D9 uses an older shader compiler that doesn't deal as well with complicated shaders.

Example failing build:

https://build.chromium.org/p/chromium.gpu.fyi/builders/Win7%20x64%20Release%20%28NVIDIA%29/builds/8417

======================================================================
FAIL: WebglConformance_conformance_glsl_misc_large_loop_compile (gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\b\s\w\irauysdc\third_party\catapult\telemetry\telemetry\testing\serially_executed_browser_test_case.py", line 155, in <lambda>
    return lambda self: based_method(self, *args)
  File "c:\b\s\w\irauysdc\content\test\gpu\gpu_tests\gpu_integration_test.py", line 78, in _RunGpuTest
    self.RunActualGpuTest(url, *args)
  File "c:\b\s\w\irauysdc\content\test\gpu\gpu_tests\webgl_conformance_integration_test.py", line 196, in RunActualGpuTest
    getattr(self, test_name)(test_path, *args[1:])
  File "c:\b\s\w\irauysdc\content\test\gpu\gpu_tests\webgl_conformance_integration_test.py", line 210, in _RunConformanceTest
    self._CheckTestCompletion()
  File "c:\b\s\w\irauysdc\content\test\gpu\gpu_tests\webgl_conformance_integration_test.py", line 206, in _CheckTestCompletion
    self.fail(self._WebGLTestMessages(self.tab))
AssertionError: Shader compilation took an unreasonably long time
FAIL Shader compilation took an unreasonably long time