diff --git a/test/test_cost_model.py b/test/test_cost_model.py
index 482262ec28582f40adfdde816cb9ec408417e388..7b24f56713dcd338cebbdcef7bb9d59c4e5cbcbf 100644
--- a/test/test_cost_model.py
+++ b/test/test_cost_model.py
@@ -342,16 +342,18 @@ class ConstantOneQBXExpansionWrangler(ConstantOneExpansionWrangler):
         center_to_tree_targets = self.geo_data.center_to_tree_targets()
         qbx_center_to_target_box = self.geo_data.qbx_center_to_target_box()
 
+        target_box_to_src_sum = {}
+        target_box_to_nsrcs = {}
+
         for ictr in global_qbx_centers:
             tgt_ibox = qbx_center_to_target_box[ictr]
 
-            ictr_tgt_start, ictr_tgt_end = center_to_tree_targets.starts[ictr:ictr+2]
+            isrc_box_start, isrc_box_end = (
+                    self.trav.neighbor_source_boxes_starts[tgt_ibox:tgt_ibox+2])
 
-            for ictr_tgt in range(ictr_tgt_start, ictr_tgt_end):
-                ctr_itgt = center_to_tree_targets.lists[ictr_tgt]
-
-                isrc_box_start, isrc_box_end = (
-                        self.trav.neighbor_source_boxes_starts[tgt_ibox:tgt_ibox+2])
+            if tgt_ibox not in target_box_to_src_sum:
+                nsrcs = 0
+                src_sum = 0
 
                 for isrc_box in range(isrc_box_start, isrc_box_end):
                     src_ibox = self.trav.neighbor_source_boxes_lists[isrc_box]
@@ -360,8 +362,22 @@ class ConstantOneQBXExpansionWrangler(ConstantOneExpansionWrangler):
                     isrc_end = (isrc_start
                             + self.tree.box_source_counts_nonchild[src_ibox])
 
-                    pot[0][ctr_itgt] += sum(src_weights[isrc_start:isrc_end])
-                    ops += isrc_end - isrc_start
+                    src_sum += sum(src_weights[isrc_start:isrc_end])
+                    nsrcs += isrc_end - isrc_start
+
+                target_box_to_src_sum[tgt_ibox] = src_sum
+                target_box_to_nsrcs[tgt_ibox] = nsrcs
+
+            src_sum = target_box_to_src_sum[tgt_ibox]
+            nsrcs = target_box_to_nsrcs[tgt_ibox]
+
+            ictr_tgt_start, ictr_tgt_end = center_to_tree_targets.starts[ictr:ictr+2]
+
+            for ictr_tgt in range(ictr_tgt_start, ictr_tgt_end):
+                ctr_itgt = center_to_tree_targets.lists[ictr_tgt]
+                pot[0][ctr_itgt] = src_sum
+
+            ops += (ictr_tgt_end - ictr_tgt_start) * nsrcs
 
         return pot, self.timing_future(ops)