Java editing slow with Flatten packages turned on

Want to share my experiance concerning slow cursor in the Java editor

I have large project in the IDE with many maven modules and when I use Project/Packages with Flatten packages, the editor becomes very slow when I introduce an error which inspection finds.

The cursor hangs or moves very slowly for many seconds. If I turn off  "Flatten packages" every thing is fine again. Turning off "inspection" also fixes the problem.

It seems like the combination "many Maven modules", "inspection" and "Flatten packages" makes the editor slow.