diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 298242f1..0032734e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -18,6 +18,18 @@ jobs:
             python-version: "3.12"
             is-primary: true
 
+    env:
+      # Collect coverage only on the primary job: it is the only one that
+      # uploads the reports and feeds the PR comment, while --cov costs
+      # ~65% extra pytest wall time (worst on Windows). Windows-only
+      # covered lines measured at 5 (diffing covered lines per file,
+      # Windows vs the primary artifact): the SIGUSR1 no-op return and the
+      # drive-letter branch of convert_project_path_to_claude_dir, both in
+      # cli.py. Those were never in the published report (non-primary
+      # coverage was computed and discarded), so this changes nothing.
+      COV: ${{ matrix.is-primary && '--cov=claude_code_log --cov-report=xml --cov-report=html --cov-report=term' || '' }}
+      COV_APPEND: ${{ matrix.is-primary && '--cov=claude_code_log --cov-append --cov-report=xml --cov-report=html --cov-report=term' || '' }}
+
     steps:
     - uses: actions/checkout@v4
       with:
@@ -38,23 +50,23 @@ jobs:
     - name: Install dependencies
       run: uv sync --all-extras --dev && uv run playwright install chromium
 
-    - name: Run unit tests with coverage
+    - name: Run unit tests (coverage on primary only)
       # -p no:playwright: unit tests don't use the browser fixtures; skipping the
       # plugin avoids a ~1s playwright import per xdist worker, which Windows pays
       # per spawned worker. See work/xdist-import-cost.md.
-      run: uv run pytest -p no:playwright -m "not (tui or browser or benchmark)" --cov=claude_code_log --cov-report=xml --cov-report=html --cov-report=term
+      run: uv run pytest -p no:playwright -m "not (tui or browser or benchmark)" ${{ env.COV }}
 
-    - name: Run TUI tests with coverage append
-      run: uv run pytest -m tui --cov=claude_code_log --cov-append --cov-report=xml --cov-report=html --cov-report=term
+    - name: Run TUI tests (coverage append on primary only)
+      run: uv run pytest -m tui ${{ env.COV_APPEND }}
 
-    - name: Run browser tests with coverage append
-      run: uv run pytest -m browser --cov=claude_code_log --cov-append --cov-report=xml --cov-report=html --cov-report=term
+    - name: Run browser tests (coverage append on primary only)
+      run: uv run pytest -m browser ${{ env.COV_APPEND }}
 
     - name: Run benchmark tests with coverage append (primary only)
       if: matrix.is-primary
       env:
         CLAUDE_CODE_LOG_DEBUG_TIMING: "1"
-      run: uv run pytest -m benchmark --cov=claude_code_log --cov-append --cov-report=xml --cov-report=html --cov-report=term -v
+      run: uv run pytest -m benchmark ${{ env.COV_APPEND }} -v
 
     - name: Upload coverage HTML report as artifact
       uses: actions/upload-artifact@v4
diff --git a/pyproject.toml b/pyproject.toml
index 2279f763..b6bc1c0f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -100,6 +100,15 @@ target-version = "py310"
 source = ["claude_code_log"]
 omit = ["*/tests/*", "*/test/*", "*/__pycache__/*", "*/venv/*", "*/.venv/*"]
 relative_files = true
+# sys.monitoring core (Py 3.12+): ~5x lower tracing overhead than the C
+# tracer (unit-test step: +67% -> +14% wall time vs no coverage). On
+# Py < 3.12 coverage warns "no-sysmon" once and falls back to the default
+# core; data and reports are unaffected. Requires coverage >= 7.9 for the
+# [run] core setting. COVERAGE_CORE env var still overrides (e.g.
+# COVERAGE_CORE=ctrace to A/B time). NOTE: sysmon cannot measure branch
+# coverage before Py 3.14 — if branch = true is ever enabled here, 3.12/
+# 3.13 silently revert to the slow tracer.
+core = "sysmon"
 
 [tool.coverage.report]
 exclude_lines = [
@@ -125,7 +134,7 @@ dev = [
     "pytest>=8.3.5",
     "pytest-asyncio>=0.25.3",
     "pytest-cov>=5.0.0",
-    "coverage[toml]>=7.6.0",
+    "coverage[toml]>=7.9",  # [run] core setting (added in 7.9); older coverage warns 'unrecognized option' and silently falls back to the slow default core
     "ruff>=0.11.2",
     "pytest-xdist[psutil]>=3.6.1",
     "pyright>=1.1.408",
diff --git a/uv.lock b/uv.lock
index a73350f5..3fc191e8 100644
--- a/uv.lock
+++ b/uv.lock
@@ -196,7 +196,7 @@ requires-dist = [
 [package.metadata.requires-dev]
 dev = [
     { name = "claude-code-log-clmail-test", editable = "test/_plugins/clmail" },
-    { name = "coverage", extras = ["toml"], specifier = ">=7.6.0" },
+    { name = "coverage", extras = ["toml"], specifier = ">=7.9" },
     { name = "pyright", specifier = ">=1.1.408" },
     { name = "pytest", specifier = ">=8.3.5" },
     { name = "pytest-asyncio", specifier = ">=0.25.3" },