diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5250a33..bfa01ed 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ on:
 
 jobs:
   build-and-test:
-    name: build-and-test (${{ matrix.cxx }}${{ matrix.asan && ', asan' || '' }})
+    name: build-and-test (${{ matrix.cxx }}${{ matrix.asan && ', asan' || '' }}${{ matrix.build_type == '' && ', no build type' || '' }})
     runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
@@ -15,6 +15,12 @@ jobs:
         cxx: [g++-14, clang++-19]
         # The sanitizer executables build in their own job, in parallel with the plain one.
         asan: [false, true]
+        build_type: [Release]
+        include:
+          # No build type: the tests compile at -O1, so its -Werror warnings are covered.
+          - cxx: clang++-19
+            asan: false
+            build_type: ""
 
     steps:
     - uses: actions/checkout@v4
@@ -34,7 +40,7 @@ jobs:
     - name: Configure
       run: |
         cmake -B build \
-          -DCMAKE_BUILD_TYPE=Release \
+          -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} \
           -DCMAKE_CXX_COMPILER=${{ matrix.cxx }} \
           -DNUMSIM_MATERIALS_BUILD_TESTS=ON \
           -DNUMSIM_SANITIZER_TESTS=${{ matrix.asan && 'ON' || 'OFF' }} \
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 4bb570b..f3ca4ca 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,5 +1,30 @@
 find_package(Threads REQUIRED)
 
+# Without a build type the tests would compile at -O0, where the uninlined expression templates make every
+# test binary ~5x larger and slower. "auto" is resolved on every configure: -O1 only if CMAKE_BUILD_TYPE is empty,
+# the generator is single-config and CMAKE_CXX_FLAGS has no -O option. Any other value is used verbatim, empty is off.
+set(NUMSIM_MATERIALS_TEST_OPT_FLAGS auto CACHE STRING
+    "Optimisation flags for the test executables (plain and sanitizer builds): auto, a flag list, or empty for none")
+get_property(_numsim_multi_config GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+if(NUMSIM_MATERIALS_TEST_OPT_FLAGS STREQUAL "auto")
+  set(_numsim_test_opt "")
+  if(NOT CMAKE_BUILD_TYPE AND NOT _numsim_multi_config
+     AND NOT CMAKE_CXX_FLAGS MATCHES "(^| )-O[0-9sgz]*( |$)")
+    set(_numsim_test_opt -O1)
+  endif()
+else()
+  separate_arguments(_numsim_test_opt UNIX_COMMAND "${NUMSIM_MATERIALS_TEST_OPT_FLAGS}")
+endif()
+
+# The optimisation flags above, on a test executable and its sanitizer variants.
+function(numsim_materials_test_options TARGET_NAME)
+  foreach(_target IN ITEMS ${TARGET_NAME} ${TARGET_NAME}_asan ${TARGET_NAME}_tsan)
+    if(TARGET ${_target})
+      target_compile_options(${_target} PRIVATE ${_numsim_test_opt})
+    endif()
+  endforeach()
+endfunction()
+
 option(NUMSIM_MATERIALS_BATCH_TESTS "Compile single-file tests several to a translation unit" ON)
 set(NUMSIM_MATERIALS_TEST_BATCH_SIZE 6 CACHE STRING "Test files per batch")
 # Tests that need their own flags or check what a lone translation unit includes.
@@ -21,6 +46,7 @@ macro(add_numsim_test TARGET_NAME)
   else()
     numsim_add_test(${TARGET_NAME} SOURCES ${ARGN}
       LINK numsim-materials::numsim-materials Threads::Threads)
+    numsim_materials_test_options(${TARGET_NAME})
     list(APPEND _numsim_test_targets ${TARGET_NAME})
   endif()
 endmacro()
@@ -124,6 +150,7 @@ while(_first LESS _nbatched)
   add_custom_target(${_name}_source DEPENDS "${_out}")
   numsim_add_test(${_name} SOURCES "${_out}" INCLUDE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}"
     LINK numsim-materials::numsim-materials Threads::Threads)
+  numsim_materials_test_options(${_name})
   list(APPEND _numsim_test_targets ${_name})
   foreach(_t ${_name} ${_name}_asan ${_name}_tsan)
     if(TARGET ${_t})