Slow editing is caused by a large number of connectors in a model. This issue has been identified in 7.3.3 and 7.3.4 versions. A fix will be in place for future releases.
In the meantime, as a workaround to speed up editing, remove the connectors from the model, finish editing and use the Reset Analysis Model command to regenerate them.
Delete all hangers in the model by:
1. Right click > select Advanced Selection
or use the keyboard shortcut Ctrl + A
2. Left click on the line Connector
3. Click the Select button followed by the OK button
4. Press the Delete key on your keyboard
When finished with edits, regenerate the connectors by going to the Analysis and Desgin tab, click on the Reset Analysis Model command.