Hi Pilou, this was a pretty bad bug, I have fixed it for the next beta.

It was partially related to undo - basically there were a lot of extra copies of objects that were taking up a lot of memory until you completed the transform command. With smaller objects this wasn't too noticable, but on a larger set of objects this could eat up quite a lot of memory and once enough memory is used so that you start swapping to disk, that usually slows things down quite a lot.

This is fixed in the next beta so that transform commands will not drain excess memory like this anymore.

Thanks for reporting this!

- Michael