So far each activity module keeps its own grades table (provided the activity has grades of course) and whenever a student or lecturer clicks the "Grades" link, the grades are collected from all these modules. This requires loading the module's lib.php and calling the function modulename_grades which compiles a table with all the grades from that module. Clearly this is a very time-consuming way to do things.
For this and other reasons the idea has been floated to create a central grades table which is simply upgraded by the activity modules whenever something changes. This is clearly a good idea, however it requires changes to all activity modules and thus should be discussed before it can be implemented.
One of the questions to be settled is the structure of the central grades table.