add note about efficiency

Author: tbrowder

doc/Language/tables.pod6

@@ -32,7 +32,7 @@ illegal.
 3. The only valid row separator characters are V<'_'>, V<'-'>, V<'+'>, V<' '>,
 V<'|'>, and V<'='>.
 
-4. Consecutive row separator lines are illegal.
+4. Consecutive row separator lines are illegal (but only generate a warning).
 
 =head1 Best Practices
 
@@ -98,6 +98,9 @@ expect a warning about short rows being filled with empty cells.
 row with the most cells. Short rows are padded on the right with
 empty cells and generate a warning.)
 
+Note that not following best practices may require more table
+processing due to additional looping over table rows.
+
 =head1 Good Tables
 
 Following are examples of valid (Good) tables (taken from the current