To face the issue, please:
- Create a batch of 500 or more dummy/test drum maps;
- Open a new project;
- Create an instrument track;
- Load the drum maps (Drum Map Setup > Functions > Load…)
- Save the project;
- Re-open the project;
- Re-close the project.
Result: closing and opening the project becomes very slow. What was a matter of seconds is now a matter of minutes.
Although technically this is not a bug, it does indicate a problematic implementation of drum maps in Cubase and thus calls for further streamlining and optimization.