diff --git a/tools/toolchain/scripts/stage8/install_sirius.sh b/tools/toolchain/scripts/stage8/install_sirius.sh
index aa93f90c0c..d227262946 100755
--- a/tools/toolchain/scripts/stage8/install_sirius.sh
+++ b/tools/toolchain/scripts/stage8/install_sirius.sh
@@ -83,8 +83,19 @@ case "$with_sirius" in
     fi
 
     if [ "$ARCH" = "x86_64" ]; then
-      SIRIUS_OPT="-O3 -DNDEBUG -mtune=native -ftree-loop-vectorize ${MATH_CFLAGS}"
-      SIRIUS_DBG="-O2 -g -mtune=native -ftree-loop-vectorize ${MATH_CFLAGS}"
+      if [ "${with_intel}" != "__DONTUSE__" ]; then
+        SIRIUS_OPT="-DNDEBUG -O2 -g ${MATH_CFLAGS}"
+        SIRIUS_DBG="-O1 -g ${MATH_CFLAGS}"
+        # SIRIUS_DBG and SIRIUS_OPT are not really considered by CMake and rather the CMAKE_BUILD_TYPE matters.
+        # The CMAKE_BUILD_TYPEs "Release" and "RelWithDebInfo" employ -O3/-O2, but already -O2 makes the SIRIUS
+        # build quite memory and time intensive. We are primarily interested here in testing SIRIUS with CP2K
+        # and thus use CMAKE_BUILD_TYPE "Debug" for testing. Switch the lines below for production.
+        # EXTRA_CMAKE_FLAGS="-DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_CXX_FLAGS= ${EXTRA_CMAKE_FLAGS}"
+        EXTRA_CMAKE_FLAGS="-DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS= ${EXTRA_CMAKE_FLAGS}"
+      else
+        SIRIUS_OPT="-O3 -DNDEBUG -mtune=native -ftree-loop-vectorize ${MATH_CFLAGS}"
+        SIRIUS_DBG="-O2 -g -mtune=native -ftree-loop-vectorize ${MATH_CFLAGS}"
+      fi
     fi
 
     pkg_install_dir="${INSTALLDIR}/sirius-${sirius_ver}"