diff --git a/CMakeLists.txt b/CMakeLists.txt
index a4571efba..c18c11658 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -204,7 +204,7 @@ if( CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
 
     set( CMAKE_CXX_FLAGS_DEBUG          "-g ${CMAKE_CXX_FLAGS_DEBUG}" )
     set( CMAKE_CXX_FLAGS_MINSIZEREL     "-Os -DNDEBUG" )
-    set( CMAKE_CXX_FLAGS_RELEASE        "-O4 -DNDEBUG" )
+    set( CMAKE_CXX_FLAGS_RELEASE        "-O3 -DNDEBUG" )
     set( CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -g" )
 
     set( CMAKE_TOOLCHAIN_PREFIX "llvm-" )