performance – Screen flickering black after upgrade from 19.04 to 21.04 (ubuntu emulated on virtualbox)

I have a macbook with 10.15.7 and virtualbox 6.1
I just updated ubuntu from 19.04 to 21.04
Now when I login into ubuntu every few (randomly) seconds I get a black screen for a while. It seems a black flickering effect.
I updated guest addition but not solved the issue.

Also I have a performance issue, the system is very slow.

Up to version 20.04 (step before 21.04) it worked properly.

Please may you help me? In this way I cannot use the system