diff --git a/src/hydra-queue-runner/hydra-queue-runner.cc b/src/hydra-queue-runner/hydra-queue-runner.cc
index 417c5202..30a1d856 100644
--- a/src/hydra-queue-runner/hydra-queue-runner.cc
+++ b/src/hydra-queue-runner/hydra-queue-runner.cc
@@ -6,6 +6,7 @@
 #include <thread>
 #include <cmath>
 #include <chrono>
+#include <algorithm>
 
 #include <pqxx/pqxx>
 
@@ -159,13 +160,7 @@ struct Machine
     unsigned int maxJobs = 1;
     float speedFactor = 1.0;
 
-    Sync<unsigned int> currentJobs;
-
-    Machine()
-    {
-        auto currentJobs_(currentJobs.lock());
-        *currentJobs_ = 0;
-    }
+    std::atomic<unsigned int> currentJobs{0};
 
     bool supportsStep(Step::ptr step)
     {
@@ -187,13 +182,11 @@ struct MachineReservation
     Machine::ptr machine;
     MachineReservation(Machine::ptr machine) : machine(machine)
     {
-        auto currentJobs_(machine->currentJobs.lock());
-        (*currentJobs_)++;
+        machine->currentJobs++;
     }
     ~MachineReservation()
     {
-        auto currentJobs_(machine->currentJobs.lock());
-        if (*currentJobs_ > 0) (*currentJobs_)--;
+        machine->currentJobs--;
     }
 };
 
@@ -284,8 +277,6 @@ public:
 
     void wakeDispatcher();
 
-    MachineReservation::ptr findMachine(Step::ptr step);
-
     void builder(Step::ptr step, MachineReservation::ptr reservation);
 
     /* Perform the given build step. Return true if the step is to be
@@ -878,49 +869,98 @@ void State::dispatcher()
 
         auto sleepUntil = system_time::max();
 
-        {
-            auto runnable_(runnable.lock());
-            printMsg(lvlDebug, format("%1% runnable builds") % runnable_->size());
+        bool keepGoing;
 
-            /* FIXME: we're holding the runnable lock too long
-               here. This could be more efficient. */
+        do {
+            /* Bail out when there are no slots left. */
+            std::vector<Machine::ptr> machinesSorted;
+            {
+                auto machines_(machines.lock());
+                machinesSorted.insert(machinesSorted.end(),
+                    machines_->begin(), machines_->end());
+            }
 
+            /* Sort the machines by a combination of speed factor and
+               available slots. Prioritise the available machines as
+               follows:
+
+               - First by load divided by speed factor, rounded to the
+                 nearest integer.  This causes fast machines to be
+                 preferred over slow machines with similar loads.
+
+               - Then by speed factor.
+
+               - Finally by load. */
+            sort(machinesSorted.begin(), machinesSorted.end(),
+                [](const Machine::ptr & a, const Machine::ptr & b) -> bool
+                {
+                    float ta = roundf(a->currentJobs / a->speedFactor);
+                    float tb = roundf(b->currentJobs / b->speedFactor);
+                    return
+                        ta != tb ? ta > tb :
+                        a->speedFactor != b->speedFactor ? a->speedFactor > b->speedFactor :
+                        a->maxJobs > b->maxJobs;
+                });
+
+            /* Find a machine with a free slot and find a step to run
+               on it. Once we find such a pair, we restart the outer
+               loop because the machine sorting will have changed. */
+            keepGoing = false;
             system_time now = std::chrono::system_clock::now();
 
-            for (auto i = runnable_->begin(); i != runnable_->end(); ) {
-                auto step = i->lock();
+            for (auto & machine : machinesSorted) {
+                // FIXME: can we lose a wakeup if a builder exits concurrently?
+                if (machine->currentJobs >= machine->maxJobs) continue;
 
-                /* Delete dead steps. */
-                if (!step) {
-                    i = runnable_->erase(i);
-                    continue;
-                }
+                auto runnable_(runnable.lock());
+                printMsg(lvlDebug, format("%1% runnable builds") % runnable_->size());
 
-                /* Skip previously failed steps that aren't ready to
-                   be retried. */
-                {
-                    auto step_(step->state.lock());
-                    if (step_->tries > 0 && step_->after > now) {
-                        if (step_->after < sleepUntil)
-                            sleepUntil = step_->after;
+                /* FIXME: we're holding the runnable lock too long
+                   here. This could be more efficient. */
+
+                for (auto i = runnable_->begin(); i != runnable_->end(); ) {
+                    auto step = i->lock();
+
+                    /* Delete dead steps. */
+                    if (!step) {
+                        i = runnable_->erase(i);
+                        continue;
+                    }
+
+                    /* Can this machine do this step? */
+                    if (!machine->supportsStep(step)) {
                         ++i;
                         continue;
                     }
+
+                    /* Skip previously failed steps that aren't ready
+                       to be retried. */
+                    {
+                        auto step_(step->state.lock());
+                        if (step_->tries > 0 && step_->after > now) {
+                            if (step_->after < sleepUntil)
+                                sleepUntil = step_->after;
+                            ++i;
+                            continue;
+                        }
+                    }
+
+                    /* Make a slot reservation and start a thread to
+                       do the build. */
+                    auto reservation = std::make_shared<MachineReservation>(machine);
+                    i = runnable_->erase(i);
+
+                    auto builderThread = std::thread(&State::builder, this, step, reservation);
+                    builderThread.detach(); // FIXME?
+
+                    keepGoing = true;
+                    break;
                 }
 
-                auto reservation = findMachine(step);
-                if (!reservation) {
-                    printMsg(lvlDebug, format("cannot execute step ‘%1%’ right now") % step->drvPath);
-                    ++i;
-                    continue;
-                }
-
-                i = runnable_->erase(i);
-
-                auto builderThread = std::thread(&State::builder, this, step, reservation);
-                builderThread.detach(); // FIXME?
+                if (keepGoing) break;
             }
-        }
+
+        } while (keepGoing);
 
         /* Sleep until we're woken up (either because a runnable build
            is added, or because a build finishes). */
@@ -944,23 +984,6 @@ void State::wakeDispatcher()
 }
 
 
-MachineReservation::ptr State::findMachine(Step::ptr step)
-{
-    auto machines_(machines.lock());
-
-    for (auto & machine : *machines_) {
-        if (!machine->supportsStep(step)) continue;
-        {
-            auto currentJobs_(machine->currentJobs.lock());
-            if (*currentJobs_ >= machine->maxJobs) continue;
-        }
-        return std::make_shared<MachineReservation>(machine);
-    }
-
-    return 0;
-}
-
-
 void State::builder(Step::ptr step, MachineReservation::ptr reservation)
 {
     bool retry = true;
@@ -1274,9 +1297,8 @@ void State::dumpStatus()
     {
         auto machines_(machines.lock());
         for (auto & m : *machines_) {
-            auto currentJobs_(m->currentJobs.lock());
             printMsg(lvlError, format("machine %1%: %2%/%3% active")
-                % m->sshName % *currentJobs_ % m->maxJobs);
+                % m->sshName % m->currentJobs % m->maxJobs);
         }
     }
 }