Small clarification

Author: pablogsal

peps/pep-0810.rst

@@ -765,8 +765,17 @@ lazy. The filter has **almost no measurable performance cost**. To measure
 this, we benchmarked importing all 278 top-level importable modules from the
 Python standard library (which transitively loads 392 total modules including
 all submodules and dependencies), then forced reification of every loaded
-module to ensure everything was fully materialized. We compared four different
-configurations:
+module to ensure everything was fully materialized.
+
+Note that these measurements establish the baseline overhead of the filter
+mechanism itself. Of course, any user-defined filter function that performs
+additional work beyond a trivial check will add overhead proportional to the
+complexity of that work. However, we expect that in practice this overhead
+will be dwarfed by the performance benefits gained from avoiding unnecessary
+imports. The benchmarks below measure the minimal cost of the filter dispatch
+mechanism when the filter function does essentially nothing.
+
+We compared four different configurations:
 
 .. list-table::
    :header-rows: 1