diff --git a/src/Reactor/Nucleus.hpp b/src/Reactor/Nucleus.hpp
index 4c08ddd..b300d9b 100644
--- a/src/Reactor/Nucleus.hpp
+++ b/src/Reactor/Nucleus.hpp
@@ -28,6 +28,12 @@
 #	undef None  // TODO(b/127920555)
 #endif
 
+// A Clang extension to determine compiler features.
+// We use it to detect Sanitizer builds (e.g. -fsanitize=memory).
+#ifndef __has_feature
+#	define __has_feature(x) 0
+#endif
+
 static_assert(sizeof(short) == 2, "Reactor's 'Short' type is 16-bit, and requires the C++ 'short' to match that.");
 static_assert(sizeof(int) == 4, "Reactor's 'Int' type is 32-bit, and requires the C++ 'int' to match that.");
 
@@ -79,6 +85,14 @@
 			this->level = Level::REACTOR_DEFAULT_OPT_LEVEL;
 		}
 #endif
+
+		// TODO(b/173257647): MemorySanitizer instrumentation produces IR which takes
+		// a lot longer to process by the machine code optimization passes. Disabling
+		// them has a negligible effect on code quality but compiles much faster.
+		if(__has_feature(memory_sanitizer))
+		{
+			this->level = Level::None;
+		}
 	}
 
 	Level getLevel() const { return level; }
diff --git a/src/Reactor/Reactor.hpp b/src/Reactor/Reactor.hpp
index 05d5925..3e22ee9 100644
--- a/src/Reactor/Reactor.hpp
+++ b/src/Reactor/Reactor.hpp
@@ -52,12 +52,6 @@
 }
 #endif
 
-// A Clang extension to determine compiler features.
-// We use it to detect Sanitizer builds (e.g. -fsanitize=memory).
-#ifndef __has_feature
-#	define __has_feature(x) 0
-#endif
-
 // Whether Reactor routine instrumentation is enabled for MSan builds.
 // TODO(b/155148722): Remove when unconditionally instrumenting for all build systems.
 #if !defined REACTOR_ENABLE_MEMORY_SANITIZER_INSTRUMENTATION