diff --git a/CMakeLists.txt b/CMakeLists.txt
index 744e10d9b..a78fb8925 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,7 +95,7 @@ else(CLANG)
 	# As CLANG is not set as a variable, we need to set it in case we have not matched another compiler.
 	set (CLANG ON)
     # Set standard flags for clang
-    set (CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -funroll-loops -O4")
+    set (CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -funroll-loops -O3")
     set (CMAKE_CXX_FLAGS "-std=c++11 -stdlib=libc++ -Wall -Werror -pedantic -Wno-unused-variable -DBOOST_RESULT_OF_USE_TR1 -DBOOST_NO_DECLTYPE")
     
     set (CMAKE_CXX_FLAGS_DEBUG "-g")