Index: cmake/UserOverride.cmake
--- cmake/UserOverride.cmake.orig
+++ cmake/UserOverride.cmake
@@ -19,7 +19,7 @@ elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
 elseif (CMAKE_CXX_COMPILER_ID MATCHES Clang)
     # clang
     set(common "-Wall -Wextra")
-    set(CMAKE_CXX_FLAGS_RELEASE_INIT "${common} -O3 -funroll-loops -DNDEBUG")
+    set(CMAKE_CXX_FLAGS_RELEASE_INIT "${common} -funroll-loops -DNDEBUG")
     set(CMAKE_CXX_FLAGS_DEBUG_INIT   "${common} -g -ggdb")
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "PGI")
     # pgcpp
