Prevent repeated call of substituter(rules) in substitute. (#1963)

Author: evetion

src/inputoutput.jl

@@ -295,7 +295,8 @@ function inputs_to_parameters!(state::TransformationState, io)
     @set! structure.var_to_diff = complete(new_var_to_diff)
     @set! structure.graph = complete(new_graph)
 
-    @set! sys.eqs = map(Base.Fix2(substitute, input_to_parameters), equations(sys))
+    @set! sys.eqs = isempty(input_to_parameters) ? equations(sys) :
+                    fast_substitute(equations(sys), input_to_parameters)
     @set! sys.states = setdiff(states(sys), keys(input_to_parameters))
     ps = parameters(sys)