Index: cmake/compiler_flags/GNU_Fortran.cmake
--- cmake/compiler_flags/GNU_Fortran.cmake.orig
+++ cmake/compiler_flags/GNU_Fortran.cmake
@@ -6,7 +6,7 @@
 # granted to it by virtue of its status as an intergovernmental organisation
 # nor does it submit to any jurisdiction.
 
-set( CMAKE_Fortran_FLAGS_RELEASE        "-O3 -DNDEBUG -funroll-all-loops -finline-functions" CACHE STRING "Fortran compiler flags for Release builds"          FORCE )
+set( CMAKE_Fortran_FLAGS_RELEASE        "-DNDEBUG -funroll-all-loops -finline-functions" CACHE STRING "Fortran compiler flags for Release builds"          FORCE )
 set( CMAKE_Fortran_FLAGS_BIT            "-O2 -DNDEBUG -fno-range-check"                      CACHE STRING "Fortran compiler flags for Bit-reproducible builds" FORCE )
 set( CMAKE_Fortran_FLAGS_DEBUG          "-O0 -g -fcheck=bounds -fbacktrace -finit-real=snan" CACHE STRING "Fortran compiler flags for Debug builds"            FORCE )
 set( CMAKE_Fortran_FLAGS_PRODUCTION     "-O2 -g"                                             CACHE STRING "Fortran compiler flags for Production builds."      FORCE )
