Refrain from parallelizing insertion in the top layer

instant-distance/src/lib.rs

@@ -314,10 +314,17 @@ where
             }
 
             let end = range.end;
+            if layer == top {
+                nodes[range].into_iter().for_each(|(_, pid)| {
+                    let node = state.zero[*pid].write();
+                    state.insert(*pid, node, layer, &layers);
+                })
+            } else {
                 nodes[range].into_par_iter().for_each(|(_, pid)| {
                     let node = state.zero[*pid].write();
                     state.insert(*pid, node, layer, &layers);
                 });
+            }
 
             // For layers above the zero layer, make a copy of the current state of the zero layer
             // with `nearest` truncated to `M` elements.