How to reproduce:
1. Open the attached project and open the "SampleScene"
2. Find the "1000" parent in the Hierarchy and expand it
3. Select all of its children and copy-paste them (Observe the time it takes to paste these objects, it should take around 1-2 sec)
4. Find the "6000" parent in the Hierarchy and expand it
5. Select all of its children and copy-paste them (Observe the time it takes to paste these objects, it takes around 40 sec)

Expected result: the performance is near-linear, for example, if it takes 1 sec to paste 1000 objects it should take around 6-7 secs to paste 6000 objects
Actual result: the performance gets worse non linearly with the more objects that are selected. Instead of 6 secs, it takes around 40 secs to paste 6000 objects. With 10000 objects it takes around 10 minutes. (See FB comment for more data)

Reproduced in: 2020.1.0a18, 2019.3.0f4, 2019.2.16f1, 2018.4.15f1

Note: only reproduces when selecting multiple objects, if only a single parent with the same amount of objects is selected the performance is fast as expected