What do you mean with slow? Normally this should not affect speed. Add dependent adds the Dialog to the dependent aggregation of your view. Then it will get access to the models on the view and also be destroyed when the view gets destroyed.
- Slow already first time or when opening again and again getting slower? --> maybe check if you instantiate multiple times
- You can also try to set the model to the dialog/popover directly by calling this._oPopover.setModel(this.getView().getModel("i18n"), "i18n") and see if it then is faster again.