Hi Jakub,
I believe the issue is moving subgroups. For example, when I convert a legend into a set of graphics I get a "parent" group element that holds all the legend graphics and a subgroup for each layer. The actual graphical elements, such as legend patch and text, are within each subgroup.
In this example, there is a parent group called "Legend" with a number of subgroups.
There are 846 individual elements within 104 subgroups in this test layout. My goal was to move all the elements to the right side of the layout page. With no groups involved, Pro took about 14 seconds to select all the elements and about 6 seconds to move. When I selected and moved the parent group "Legend" the results were about 5 seconds to select and 8 seconds to move. Not lightning fast, but not horrible.
However, if I move selected subgroups ArcGIS Pro's performance is indeed poor. In my example I selected each of the 104 subgroups (which took about 5 seconds).
OK, so far. But when I moved them - it took more than 5 minutes for Pro to finish the move!
Ugh!
This is a bad bug. I'll get this into the queue for immediate investigation.
Thank you for helping us identify the issue.
Tom