Refs #35936 -- Avoided field placeholder lookup for each value inserted.

By building the list of placeholders for each inserted fields once it
doesn't have to be looked up for each inserted rows twice.

The query_values_10000.benchmark.QueryValues10000.time_query_values_10000 ASV
benchmark showed a 5% speed up for 10k items on SQLite for a single field
insertion. Larger performance gains are expected when more fields are involved.

django/db/models/sql/compiler.py

@@ -1688,7 +1688,7 @@ class SQLInsertCompiler(SQLCompiler):
     returning_fields = None
     returning_params = ()
 
-    def field_as_sql(self, field, val):
+    def field_as_sql(self, field, get_placeholder, val):
         """
         Take a field and a value intended to be saved on that field, and
         return placeholder SQL and accompanying params. Check for raw values,
@@ -1703,10 +1703,10 @@ def field_as_sql(self, field, val):
         elif hasattr(val, "as_sql"):
             # This is an expression, let's compile it.
             sql, params = self.compile(val)
-        elif hasattr(field, "get_placeholder"):
+        elif get_placeholder is not None:
             # Some fields (e.g. geo fields) need special munging before
             # they can be inserted.
-            sql, params = field.get_placeholder(val, self, self.connection), [val]
+            sql, params = get_placeholder(val, self, self.connection), [val]
         else:
             # Return the common case for the placeholder
             sql, params = "%s", [val]
@@ -1775,8 +1775,12 @@ def assemble_as_sql(self, fields, value_rows):
 
         # list of (sql, [params]) tuples for each object to be saved
         # Shape: [n_objs][n_fields][2]
+        get_placeholders = [getattr(field, "get_placeholder", None) for field in fields]
         rows_of_fields_as_sql = (
-            (self.field_as_sql(field, v) for field, v in zip(fields, row))
+            (
+                self.field_as_sql(field, get_placeholder, value)
+                for field, get_placeholder, value in zip(fields, get_placeholders, row)
+            )
             for row in value_rows
         )