Hi Klaus,
I found your post, by googling for the same problem.
Actually I have a Kubuntu 18.04.

For me the problem is SweetHome 3D version 6.0 (I downloaded it from the SweetHome 3D Website).

Then I tried out the recent Sweet Home 3D version 5.7, which is packed in the Ubuntu Distribution Repository.
And everything is fast.

Unluckily I did not find the 5.7 version on the official page any more, since the distributed version of Sweet Home 3D does not support video recording.

Right now I try to get along with both versions installed.
Best regards,
Bastian