The comments view is not aware of the type of entity that a given query is interested in. It is designed so that you could do a query like select * from comment and it would properly suppress any and all comments that should not be publicly viewable. To accomplish this, the view's WHERE clause includes 9 correlated subqueries against 8 other tables.
Most uses of the comment table would only be interested in the comments that go with rows from a single "entity" table (for example ipblocks). Having additional views that are restricted to a single entity type should provide some performance improvements to queries that know to use a specialized view.
The actor view has this identical issue as well.