diff --git a/.github/workflows/build_orca.yml b/.github/workflows/build_orca.yml
index f1fb29c46b..e51cb2e37a 100644
--- a/.github/workflows/build_orca.yml
+++ b/.github/workflows/build_orca.yml
@@ -117,12 +117,10 @@ jobs:
         run: |
           echo "CMAKE_C_COMPILER_LAUNCHER=ccache" >> "$GITHUB_ENV"
           echo "CMAKE_CXX_COMPILER_LAUNCHER=ccache" >> "$GITHUB_ENV"
-          # Headers a fresh checkout has just written, and the few files that
-          # use __DATE__ or __TIME__.
-          echo "CCACHE_SLOPPINESS=time_macros,include_file_mtime,include_file_ctime" >> "$GITHUB_ENV"
-          # Clang rebuilds the precompiled header with a fresh timestamp on
-          # every run, so everything that includes it would miss.
-          echo "ORCA_EXTRA_BUILD_ARGS=-DSLIC3R_PCH=OFF" >> "$GITHUB_ENV"
+          # Headers a fresh checkout has just written, the few files that
+          # use __DATE__ or __TIME__, and the precompiled header, whose
+          # macros ccache cannot see.
+          echo "CCACHE_SLOPPINESS=pch_defines,time_macros,include_file_mtime,include_file_ctime" >> "$GITHUB_ENV"
           # The restored directory carries the previous run's counters.
           ccache -z
 
diff --git a/cmake/modules/PrecompiledHeader.cmake b/cmake/modules/PrecompiledHeader.cmake
index 7ef80aacff..7d8b3a5603 100644
--- a/cmake/modules/PrecompiledHeader.cmake
+++ b/cmake/modules/PrecompiledHeader.cmake
@@ -256,6 +256,13 @@ function(add_precompiled_header _target _input)
     message(STATUS "Adding precompiled header ${_input} to target ${_target}.")
     target_precompile_headers(${_target} PRIVATE ${_input})
 
+    # Clang records the modification time of every input in the precompiled
+    # header, which makes it differ between two checkouts of the same source
+    # and defeats a compiler cache. The build system already rebuilds the
+    # header when an input changes.
+    target_compile_options(${_target} PRIVATE
+        "$<$<CXX_COMPILER_ID:Clang,AppleClang>:SHELL:-Xclang -fno-pch-timestamp>")
+
     get_target_property(_sources ${_target} SOURCES)
     list(FILTER _sources INCLUDE REGEX ".*\\.mm?")