diff --git a/src/ds/seqset.h b/src/ds/seqset.h
index 13f9faf..da5eb46 100644
--- a/src/ds/seqset.h
+++ b/src/ds/seqset.h
@@ -159,5 +159,13 @@ namespace snmalloc
         v.end = &(item->next);
       }
     }
+
+    /**
+     * Peek at next element in the set.
+     */
+    SNMALLOC_FAST_PATH const T* peek()
+    {
+      return v.head;
+    }
   };
 } // namespace snmalloc
diff --git a/src/mem/corealloc.h b/src/mem/corealloc.h
index 89adfc8..f5f03f8 100644
--- a/src/mem/corealloc.h
+++ b/src/mem/corealloc.h
@@ -689,11 +689,16 @@ namespace snmalloc
       {
 #ifdef SNMALLOC_CHECK_CLIENT
         // Occassionally don't use the last list.
-        if (
-          SNMALLOC_UNLIKELY(alloc_classes[sizeclass].length == 1) &&
-          (entropy.next_bit() == 0))
+        if (SNMALLOC_UNLIKELY(alloc_classes[sizeclass].length == 1))
         {
-          return small_alloc_slow<zero_mem>(sizeclass, fast_free_list);
+          // If the slab has a lot of free space, then we shouldn't allocate a
+          // new slab.
+          auto min = alloc_classes[sizeclass]
+                       .available.peek()
+                       ->free_queue.min_list_length();
+          if ((min * 2) < threshold_for_waking_slab(sizeclass))
+            if (entropy.next_bit() == 0)
+              return small_alloc_slow<zero_mem>(sizeclass, fast_free_list);
         }
 #endif
 
diff --git a/src/mem/freelist.h b/src/mem/freelist.h
index d2e5a18..fbd8509 100644
--- a/src/mem/freelist.h
+++ b/src/mem/freelist.h
@@ -740,6 +740,20 @@ namespace snmalloc
         UNUSED(domesticate);
 #endif
       }
+
+      /**
+       * Returns length of the shorter free list.
+       *
+       * This method is only usable if the free list is adding randomisation
+       * as that is when it has two lists.
+       */
+      template<bool RANDOM_ = RANDOM>
+      [[nodiscard]] std::enable_if_t<RANDOM_, size_t> min_list_length() const
+      {
+        static_assert(RANDOM_ == RANDOM, "Don't set SFINAE parameter!");
+
+        return length[0] < length[1] ? length[0] : length[1];
+      }
     };
   } // namespace freelist
 } // namespace snmalloc