diff --git a/src/hydra-queue-runner/build-remote.cc b/src/hydra-queue-runner/build-remote.cc
index e25da38d..7e307c75 100644
--- a/src/hydra-queue-runner/build-remote.cc
+++ b/src/hydra-queue-runner/build-remote.cc
@@ -398,6 +398,7 @@ private:
 };
 
 void State::buildRemote(ref<Store> destStore,
+    std::unique_ptr<MachineReservation> reservation,
     ::Machine::ptr machine, Step::ptr step,
     const ServeProto::BuildOptions & buildOptions,
     RemoteResult & result, std::shared_ptr<ActiveStep> activeStep,
@@ -546,6 +547,14 @@ void State::buildRemote(ref<Store> destStore,
         }
         SemaphoreReleaser releaser(&localWorkThrottler);
 
+        /* Once we've started copying outputs, release the machine reservation
+         * so further builds can happen. We do not release the machine earlier
+         * to avoid situations where the queue runner is bottlenecked on
+         * copying outputs and we end up building too many things that we
+         * haven't been able to allow copy slots for. */
+        reservation.reset();
+        wakeDispatcher();
+
         StorePathSet outputs;
         for (auto & [_, realisation] : buildResult.builtOutputs)
             outputs.insert(realisation.outPath);
diff --git a/src/hydra-queue-runner/builder.cc b/src/hydra-queue-runner/builder.cc
index 018215a5..ff0634b1 100644
--- a/src/hydra-queue-runner/builder.cc
+++ b/src/hydra-queue-runner/builder.cc
@@ -16,7 +16,7 @@ void setThreadName(const std::string & name)
 }
 
 
-void State::builder(MachineReservation::ptr reservation)
+void State::builder(std::unique_ptr<MachineReservation> reservation)
 {
     setThreadName("bld~" + std::string(reservation->step->drvPath.to_string()));
 
@@ -35,22 +35,20 @@ void State::builder(MachineReservation::ptr reservation)
             activeSteps_.lock()->erase(activeStep);
         });
 
+        std::string machine = reservation->machine->storeUri.render();
+
         try {
             auto destStore = getDestStore();
-            res = doBuildStep(destStore, reservation, activeStep);
+            // Might release the reservation.
+            res = doBuildStep(destStore, std::move(reservation), activeStep);
         } catch (std::exception & e) {
             printMsg(lvlError, "uncaught exception building ‘%s’ on ‘%s’: %s",
-                localStore->printStorePath(reservation->step->drvPath),
-                reservation->machine->storeUri.render(),
+                localStore->printStorePath(activeStep->step->drvPath),
+                machine,
                 e.what());
         }
     }
 
-    /* Release the machine and wake up the dispatcher. */
-    assert(reservation.unique());
-    reservation = 0;
-    wakeDispatcher();
-
     /* If there was a temporary failure, retry the step after an
        exponentially increasing interval. */
     Step::ptr step = wstep.lock();
@@ -72,11 +70,11 @@ void State::builder(MachineReservation::ptr reservation)
 
 
 State::StepResult State::doBuildStep(nix::ref<Store> destStore,
-    MachineReservation::ptr reservation,
+    std::unique_ptr<MachineReservation> reservation,
     std::shared_ptr<ActiveStep> activeStep)
 {
-    auto & step(reservation->step);
-    auto & machine(reservation->machine);
+    auto step(reservation->step);
+    auto machine(reservation->machine);
 
     {
         auto step_(step->state.lock());
@@ -211,7 +209,7 @@ State::StepResult State::doBuildStep(nix::ref<Store> destStore,
 
         try {
             /* FIXME: referring builds may have conflicting timeouts. */
-            buildRemote(destStore, machine, step, buildOptions, result, activeStep, updateStep, narMembers);
+            buildRemote(destStore, std::move(reservation), machine, step, buildOptions, result, activeStep, updateStep, narMembers);
         } catch (Error & e) {
             if (activeStep->state_.lock()->cancelled) {
                 printInfo("marking step %d of build %d as cancelled", stepNr, buildId);
diff --git a/src/hydra-queue-runner/dispatcher.cc b/src/hydra-queue-runner/dispatcher.cc
index 11db0071..d3e145de 100644
--- a/src/hydra-queue-runner/dispatcher.cc
+++ b/src/hydra-queue-runner/dispatcher.cc
@@ -288,7 +288,7 @@ system_time State::doDispatch()
                 /* Make a slot reservation and start a thread to
                    do the build. */
                 auto builderThread = std::thread(&State::builder, this,
-                    std::make_shared<MachineReservation>(*this, step, mi.machine));
+                    std::make_unique<MachineReservation>(*this, step, mi.machine));
                 builderThread.detach(); // FIXME?
 
                 keepGoing = true;
diff --git a/src/hydra-queue-runner/state.hh b/src/hydra-queue-runner/state.hh
index 12aead40..edfad4fb 100644
--- a/src/hydra-queue-runner/state.hh
+++ b/src/hydra-queue-runner/state.hh
@@ -400,7 +400,6 @@ private:
 
     struct MachineReservation
     {
-        typedef std::shared_ptr<MachineReservation> ptr;
         State & state;
         Step::ptr step;
         Machine::ptr machine;
@@ -550,16 +549,17 @@ private:
 
     void abortUnsupported();
 
-    void builder(MachineReservation::ptr reservation);
+    void builder(std::unique_ptr<MachineReservation> reservation);
 
     /* Perform the given build step. Return true if the step is to be
        retried. */
     enum StepResult { sDone, sRetry, sMaybeCancelled };
     StepResult doBuildStep(nix::ref<nix::Store> destStore,
-        MachineReservation::ptr reservation,
+        std::unique_ptr<MachineReservation> reservation,
         std::shared_ptr<ActiveStep> activeStep);
 
     void buildRemote(nix::ref<nix::Store> destStore,
+        std::unique_ptr<MachineReservation> reservation,
         Machine::ptr machine, Step::ptr step,
         const nix::ServeProto::BuildOptions & buildOptions,
         RemoteResult & result, std::shared_ptr<ActiveStep> activeStep,