I don't get why a DynamicGroup manages a list of methods and not a list
of classes like DyanmicClassGroup.

Me for the title.

And to answer shortly, to have groups with a finer grained definition.

If Nautilus tries to create the class list from a DyanicGroup group it takes
awful long time if this Groups have more than 20-30 different classes

Then maybe the correct kind of group should be used.