diff --git a/app_new/c4/routes.py b/app_new/c4/routes.py
index 207aa8d..b86b87b 100644
--- a/app_new/c4/routes.py
+++ b/app_new/c4/routes.py
@@ -3,6 +3,7 @@ from datetime import date
 
 from flask import Blueprint, flash, redirect, render_template, request, url_for
 from flask_login import current_user, login_required
+from sqlalchemy.orm import joinedload, selectinload
 
 from ..core.authorization import permission_required
 from ..core.models import (
@@ -67,10 +68,14 @@ def new_contract():
 @login_required
 @permission_required("contract.view")
 def contract_detail(contract_id):
-    contract = PhotocopierContract.query.get_or_404(contract_id)
+    contract = (PhotocopierContract.query.options(
+        joinedload(PhotocopierContract.supplier),
+        selectinload(PhotocopierContract.periods),
+        selectinload(PhotocopierContract.assignments).selectinload(ContractEquipmentAssignment.equipment),
+    ).get_or_404(contract_id))
     periods = contract.periods
-    data = {period.id: calculate_contract_usage(period) for period in periods}
     projections = {period.id: project_contract_usage(period) for period in periods}
+    data = {period.id: projections[period.id]["usage"] for period in periods}
     return render_template("c4/contract_detail.html", contract=contract, periods=periods, usage=data, projections=projections)
 
 
diff --git a/app_new/core/services/c4_service.py b/app_new/core/services/c4_service.py
index b86f179..29bf0a1 100644
--- a/app_new/core/services/c4_service.py
+++ b/app_new/core/services/c4_service.py
@@ -133,6 +133,24 @@ def _meter_consumption(meter, start_date, end_date):
     return total, quality, len(readings)
 
 
+def _meter_consumption_from_rows(rows, start_date, end_date):
+    """Même calcul que ``_meter_consumption`` à partir d'un lot déjà chargé."""
+    start_dt = datetime.combine(start_date, datetime.min.time())
+    end_dt = datetime.combine(end_date, datetime.max.time())
+    relevant = [row for row in rows if row.reading_date <= end_dt]
+    before = next((row for row in reversed(relevant) if row.reading_date < start_dt), None)
+    inside = [row for row in relevant if start_dt <= row.reading_date <= end_dt]
+    ordered = ([before] if before else []) + inside
+    total = 0.0
+    previous = None
+    for reading in ordered:
+        if previous and not reading.is_reset and not previous.is_reset and reading.value >= previous.value:
+            total += reading.value - previous.value
+        previous = reading
+    quality = "EXACT" if before else ("ESTIMATED" if len(ordered) >= 2 else "PARTIAL")
+    return total, quality, len(ordered)
+
+
 def calculate_contract_usage(period):
     result = {"bw": 0.0, "color": 0.0, "quality": "EXACT", "machines": 0}
     assignments = ContractEquipmentAssignment.query.filter(
@@ -140,12 +158,23 @@ def calculate_contract_usage(period):
         ContractEquipmentAssignment.entry_date <= period.end_date,
         or_(ContractEquipmentAssignment.exit_date.is_(None), ContractEquipmentAssignment.exit_date >= period.start_date),
     ).all()
+    equipment_ids = {assignment.equipment_id for assignment in assignments}
+    meters = Meter.query.filter(Meter.equipment_id.in_(equipment_ids)).all() if equipment_ids else []
+    meter_by_equipment = {}
+    for meter in meters:
+        meter_by_equipment.setdefault(meter.equipment_id, []).append(meter)
+    meter_ids = [meter.id for meter in meters]
+    reading_rows = (MeterReading.query.filter(MeterReading.meter_id.in_(meter_ids))
+                    .order_by(MeterReading.meter_id, MeterReading.reading_date, MeterReading.id).all()) if meter_ids else []
+    readings_by_meter = {}
+    for reading in reading_rows:
+        readings_by_meter.setdefault(reading.meter_id, []).append(reading)
     for assignment in assignments:
         result["machines"] += 1
         start = max(period.start_date, assignment.entry_date)
         end = min(period.end_date, assignment.exit_date or period.end_date)
-        for meter in Meter.query.filter_by(equipment_id=assignment.equipment_id).all():
-            amount, quality, _ = _meter_consumption(meter, start, end)
+        for meter in meter_by_equipment.get(assignment.equipment_id, []):
+            amount, quality, _ = _meter_consumption_from_rows(readings_by_meter.get(meter.id, []), start, end)
             result[_meter_role(meter)] += amount
             if quality != "EXACT":
                 result["quality"] = "PARTIAL" if result["quality"] == "EXACT" else result["quality"]
diff --git a/tests/integration/test_checkpoint_c4.py b/tests/integration/test_checkpoint_c4.py
index c4c5fa7..f1bdfb1 100644
--- a/tests/integration/test_checkpoint_c4.py
+++ b/tests/integration/test_checkpoint_c4.py
@@ -1,4 +1,5 @@
 from datetime import date, datetime, timedelta, timezone
+from time import perf_counter
 
 import pytest
 
@@ -10,7 +11,7 @@ from app_new.core.models import (
 from app_new.core.services.c4_service import (
     C4DomainError, assign_equipment_to_contract, calculate_contract_usage,
     create_consumable_order, create_contract, estimate_consumable_lifetime,
-    evaluate_quota_alert, receive_consumable_order, record_consumable_issue, replace_contract_equipment,
+    evaluate_quota_alert, project_contract_usage, receive_consumable_order, record_consumable_issue, replace_contract_equipment,
 )
 
 
@@ -146,3 +147,39 @@ def test_c4_contract_http_and_permissions(authenticated_client):
     assert "Contrats photocopieurs" in response.get_data(as_text=True)
     response = authenticated_client.get("/c4/consumables")
     assert response.status_code == 200
+
+
+def test_c4_performance_paths(authenticated_client, app):
+    """Mesure les chemins C4 sans confondre le coût d'un GET et d'un calcul métier."""
+    with app.app_context():
+        supplier = _company("TEST_UI_C4_PERF_SUPPLIER")
+        contract = create_contract(supplier_id=supplier.id, name="TEST_UI_C4_PERF_CONTRACT", start_date=date(2026, 1, 1), end_date=date(2027, 12, 31), anniversary_month=1, anniversary_day=1)
+        period = contract.periods[0]
+        period.quota_bw = 10000; period.quota_color = 5000
+        machines = [_equipment(f"TEST_UI_C4_PERF_MACHINE_{i}") for i in range(5)]
+        for machine in machines:
+            assign_equipment_to_contract(contract=contract, equipment=machine, entry_date=period.start_date, commit=False)
+            bw = _meter(machine, f"N&B {machine.name}", "N&B")
+            color = _meter(machine, f"Couleur {machine.name}", "Couleur")
+            for index in range(1, 8):
+                _reading(bw, index * 100, date(2026, index, 1))
+                _reading(color, index * 40, date(2026, index, 1))
+        consumable = Consumable(name="TEST_UI_C4_PERF_TONER", quantity=10, min_quantity=2)
+        db.session.add(consumable); db.session.flush()
+        for machine in machines:
+            db.session.add(EquipmentConsumable(equipment=machine, consumable=consumable))
+        db.session.commit()
+        from sqlalchemy import event
+        counts = {"n": 0}
+        def count(_conn, _cursor, _statement, _params, _ctx, _executemany): counts["n"] += 1
+        event.listen(db.engine, "before_cursor_execute", count)
+        started = perf_counter(); usage = calculate_contract_usage(period); projection = project_contract_usage(period); service_seconds = perf_counter() - started
+        service_sql = counts["n"]
+        counts["n"] = 0
+        started = perf_counter(); response = authenticated_client.get(f"/c4/contracts/{contract.id}"); contract_seconds = perf_counter() - started; contract_sql = counts["n"]
+        counts["n"] = 0
+        started = perf_counter(); response_consumables = authenticated_client.get(f"/c4/consumables/{consumable.id}"); consumable_seconds = perf_counter() - started; consumable_sql = counts["n"]
+        event.remove(db.engine, "before_cursor_execute", count)
+        assert response.status_code == 200 and response_consumables.status_code == 200
+        assert usage["machines"] == 5 and projection["usage"]["bw"] > 0
+        print(f"C4_PERF service_seconds={service_seconds:.4f} service_queries={service_sql} contract_seconds={contract_seconds:.4f} contract_queries={contract_sql} consumable_seconds={consumable_seconds:.4f} consumable_queries={consumable_sql}")