If you loaded several models in your workspace (for example after you have used the Modeling - Load UML Models command), you might experience that some model editing operations become somewhat slower. For example, there could be a delay of 1-2 seconds when creating a new class in the model. If this happens, you can improve editing performance by turning off validation constraints that execute in live (also known as automatic) mode.
Go to Preferences - Model Validation and turn off the preference Include live warnings. Next, go to the Constraints page just below it and disable as many of the validation constraints as possible. Disabling a constraint prevents it from executing both in live and batch mode. It is therefore a good idea to enable validation constraints sometimes to ensure that the model complies with both standard UML constraints and RT specific constraints. It is not possible to prevent a constraint from executing only in live mode, which is what is required to improve editing performance.