diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 56864f3..03464c5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,6 +21,10 @@ jobs:
       PYTHONOS_QEMU_ACCEL: tcg
       PYTHONOS_GUI_BOOT_TIMEOUT: "90"
       PYTHONOS_AUDIO_BOOT_TIMEOUT: "90"
+      # Both empty except on the macOS Intel runner, where the readers fall
+      # back to their defaults.
+      PYTHONOS_BUILD_JOBS: ${{ matrix.build_jobs }}
+      PYTHONOS_GUI_COMMAND_TIMEOUT: ${{ matrix.gui_command_timeout }}
     strategy:
       fail-fast: false
       matrix:
@@ -41,6 +45,12 @@ jobs:
           # replace with macos-latest (Apple Silicon) without validating Docker.
           - name: macos-x86_64
             runner: macos-15-intel
+            # One compiler at a time, and a longer GUI command budget: this
+            # runner builds and boots inside a QEMU-backed colima VM, so both
+            # the cross compiler and the guest are markedly slower than the
+            # Linux hosts.
+            build_jobs: "1"
+            gui_command_timeout: "240"
             qemu_bin: qemu-system-x86_64
             artifact: pythonos-macos-x86_64
             artifact_path: build/pythonos.iso
diff --git a/GNUmakefile b/GNUmakefile
index 2217773..14a5b29 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -464,7 +464,8 @@ docker-build:
 # ── CPython library (slow, cached — only rebuild if missing) ─────────────────
 
 $(LIBPYTHON): tools/setup_cpython.sh .docker-image
-	$(DOCKER_RUN) -e PYTHONOS_FREE_THREADING=$(PYTHONOS_FREE_THREADING) $(DOCKER_IMG) \
+	$(DOCKER_RUN) -e PYTHONOS_FREE_THREADING=$(PYTHONOS_FREE_THREADING) \
+	  -e PYTHONOS_BUILD_JOBS=$(PYTHONOS_BUILD_JOBS) $(DOCKER_IMG) \
 	  ./tools/setup_cpython.sh --build
 
 # ── Source file sets ($(wildcard) is evaluated by Make, not a subshell) ──────
diff --git a/tests/gui_smoke_test.py b/tests/gui_smoke_test.py
index 4c0cdb0..a064e57 100644
--- a/tests/gui_smoke_test.py
+++ b/tests/gui_smoke_test.py
@@ -31,9 +31,26 @@
     QemuMonitor, parse_ppm, sample_pixel, color_close,
 )
 
+def _env_seconds(name: str, default: float) -> float:
+    """Read a timeout from the environment, falling back on anything unusable.
+
+    A workflow that sets this from an unset matrix key passes an empty string
+    rather than leaving the variable out, and bare float("") raises. Treat
+    empty, non-numeric and non-positive values as "not configured" so a typo
+    cannot turn a timeout into a crash -- or into zero.
+    """
+    raw = os.environ.get(name, "")
+    try:
+        value = float(raw)
+    except ValueError:
+        return default
+    return value if value > 0 else default
+
+
 ISO = sys.argv[1] if len(sys.argv) > 1 else "build/pythonos.iso"
 PORT = int(os.environ.get("PYTHONOS_GUI_HOST_PORT", "5559"))
-BOOT_TIMEOUT = float(os.environ.get("PYTHONOS_GUI_BOOT_TIMEOUT", "30"))
+BOOT_TIMEOUT = _env_seconds("PYTHONOS_GUI_BOOT_TIMEOUT", 30.0)
+COMMAND_TIMEOUT = _env_seconds("PYTHONOS_GUI_COMMAND_TIMEOUT", 15.0)
 
 SERIAL_LOG = "/tmp/pythonos-gui-smoke.log"
 MONITOR_SOCK = "/tmp/pythonos-gui-smoke.mon.sock"
@@ -84,7 +101,7 @@ def _send(s: socket.socket, line: str, wait: float = 2.5) -> str:
     chunks = []
     # Cross-architecture TCG can exceed the old 1-2.5 second budgets. This is
     # a deadline, not a sleep: successful commands still return immediately.
-    deadline = time.time() + max(wait, float(os.environ.get("PYTHONOS_GUI_COMMAND_TIMEOUT", "15")))
+    deadline = time.time() + max(wait, COMMAND_TIMEOUT)
     marker_seen = False
     complete = False
     while time.time() < deadline:
diff --git a/tools/setup_cpython.sh b/tools/setup_cpython.sh
index 588291f..5cdcc6f 100755
--- a/tools/setup_cpython.sh
+++ b/tools/setup_cpython.sh
@@ -404,7 +404,16 @@ if [[ "$BUILD_REQUESTED" == "1" ]]; then
     #   PYTHON_FOR_FREEZE            — use python3.14 for non-bootstrap freeze
     # -W Makefile -W Modules/config.c: additionally tell make both are up-to-date
     # so the self-regen recipe is skipped if possible.
-    make -j"$(nproc)" "$LIBPYTHON_ARCHIVE" \
+    # PYTHONOS_BUILD_JOBS caps parallelism for hosts that cannot feed nproc
+    # compilers at once. The macOS Intel runner builds inside a 6 GB colima VM,
+    # where three concurrent cc1 processes on CPython's larger generated
+    # translation units crash the compiler -- a different file each time.
+    jobs="${PYTHONOS_BUILD_JOBS:-$(nproc)}"
+    case "$jobs" in
+        ''|*[!0-9]*|0) jobs="$(nproc)" ;;
+    esac
+    echo "[setup_cpython] building libpython with -j$jobs"
+    make -j"$jobs" "$LIBPYTHON_ARCHIVE" \
         -W Makefile \
         -W Modules/config.c \
         'FREEZE_MODULE_BOOTSTRAP=python3.14 ./Programs/_freeze_module.py' \