The drivers actually contain a broken LAG implementation, so the current state is even worse than just being slow.
It is supported with the package luci-proto-bonding. Unfortunately, it allows only configuration as an interface which needs to have an IP address and thus can't be part of a bridge. This is not just a limitation of LuCI. To be actually useful, netifd would have to be changed to support configuration as a device instead. There is an open issue about this (which seems to be associated with the wrong repository).