@Marcus Papathoma (machieb) What are your hardware specs, on my desktop machine ( i7@3.4GHz, 16GB RAM ) using git commit: blender-2.80.0-git.80ffa73b3f87 it takes around 10 seconds?

@John (jfraser): I testest it at work. My computer there is a AMD Threadripper 1950X 16Cores@3,4GHz 64GB Ram using the build from yesterday.
Now I tested it at home. At my desktop Intel Xeon E5-2697v3 (2x14Cores@2,6Ghz 128GB RAM) it takes about 12seconds deleting the hierarchy using the build from today and the scene from above.

I opend this ticket because deleting something takes too much time, especially in larger scenes. Maybe the task is only single threaded?
I also started another ticket with the same scene. There you can see that undo takes to much time. The undo of a cube rotation takes at a lot of time. I think it should be investigated.
At work we use other 3D packages where an undo with the same poly and objectcount takes only a few seconds.

I would say that we can do much better here, and this is something that is worth improving. But the current behavior is expected given the current code architecture, and so outside the scope of a bug report.

@Bastien Montagne (mont29) : Thanks for your answer. As I wrote in the description, I tried to create a simple scene that mimics the scenes we use every day in our company. I can´t send you a 2GB blend file with client data. Therefore a used the default cube subdivided it and used the array modifier to create an amount of objects, to get an object count like in our scenes. The other rows of cubes were only linked copies, to hold the filesize low.
As brecht wrote it is apparently a limitation of the blender code architecture, that deleting takes a lot of time, if you have a high object and polycount in a scene.

@Brecht Van Lommel (brecht) : Thanks for your answer. I hope in the future you can improve that part of blender!

rB32dc085289ac adds a hidden experimental speed-up for that kind of massive deletion (about 20% in cased described here, this will vary a lot depending on amount of datablocks in the .blend file, and amount of collections etc.). To test it, you'll have to set debug value to 666 (F3, search for Debug Menu operator, and type 666 in numfield of the pop-up that appears).

Again, this is experimental, some quick tests seem to show it's working, but hard to tell with all the complicated cases of real life files. So use (test) it with precautions, assuming it might corrupted your .blend or crash blender, for now. ;)

@Bastien Montagne (mont29) : Thank you very much! I will test it!

@Bastien Montagne (mont29): Today I´ve tested your hidden experimental speed-up for delete hierarchy with two of our production files.
Scene 1:
Size: 2,12GB
18.457,133 Faces
18.457,134 Tris
6590 Objekte
4043 Objekte deleted

normal delete hierarchy: 28seconds
experimental speed-up: 20seconds
speedup: 28,57%

Scene 2:
Size: 2,13GB
19.601,078 Faces
19.601,788 Tris
8546 Objekte
4033 Objekte deleted

normal delete hierarchy: 48seconds
experimental speed-up: 34seconds
speedup: 29,16%

There were no links in these files only geometry in empties and collections.
I saved an opened the files no crash.
Tell me if I can help testing further and if you need more information.

It is a very good speedup. Thanks for your effort!

@Marcus Papathoma (machieb) Thanks for testing! Ideally, doing some tests with more complex production files (involving armatures and/or linked data) would also be nice… Did some here, but I cannot cover all cases. :)

Otherwise, I think that within a few days I will just invert the debug value effect (i.e. enable new code by default), and we will see. Better do that soon enough so that it get well tested while we are still in beta phase. Will also investigate adding this to other delete operations (when they involve a significant amount of IDs).

Note that number of faces should not have any effect here, what is slow is going over all IDs (objects, but also collections, etc.) to check and remove references to deleted ones.
Also related, speed-up will become more and more important when you delete a bigger amount of IDs (i.e. if you delete 95% of your objects at once, it could be 2-3 times quicker, while if you only delete 5% of the objects, speed-up will be marginal at best).

@Bastien Montagne (mont29) : I tested another production file with linked data. Sorry we don´t use amatures in our big production files, so I couldn´t test that usecase.
In the tested scene there are 8893 objects and I deleted more than half of them (4850objects). Without the optimization it took 1min:58sec and with your new code 1min:26sec.
I saved and reopened the file and couldn´d find any problems or issues.