[BUG] Performance tunning of project, convert, flatten, etc.

**Describe the bug**

The performance of those operators seems blackboxes for both us developers and for end-users. That's looks bad, as we at least should give users (and us developers, as we offen be asked by users) the intuition about how those overhead looks like compared with loading a graph.

Seems that we never measure such overhead and haven't tried to optimize yet.

**Additional context**
Add any other context about the problem here.