fixup @nilsdeppe

src/Elliptic/FirstOrderOperator.hpp

@@ -71,7 +71,9 @@ struct FirstOrderSourcesImpl<Dim, tmpl::list<PrimalFields...>,
               get<auxiliary_field_tag>(vars);
         });
     // Call into the sources computer to set primal field sources and possibly
-    // adjust auxiliary field sources
+    // adjust auxiliary field sources. We pass the volume fluxes instead of the
+    // auxiliary variables to the source computer as an optimization so they
+    // don't have to be re-computed.
     SourcesComputer::apply(
         make_not_null(&get<::Tags::Source<PrimalFields>>(*sources))...,
         make_not_null(&get<::Tags::Source<AuxiliaryFields>>(*sources))...,
@@ -125,6 +127,11 @@ auto first_order_fluxes(const Variables<VarsTags>& vars,
  * \brief Compute the sources \f$S(u)\f$ for the first-order formulation of
  * elliptic systems.
  *
+ * This function takes the `fluxes` as an argument in addition to the variables
+ * as an optimization. The fluxes will generally be computed before the sources
+ * anyway, so we pass them to the source computers to avoid having to re-compute
+ * them for source-terms that have the same form as the fluxes.
+ *
  * \see `elliptic::first_order_operator`
  */
 template <size_t Dim, typename PrimalFields, typename AuxiliaryFields,