There are a couple of issues we are working to resolve which typically have to do large spaces and/or large page trees.
This is a possible work around (we are working to fix this problem and have improved performance with large spaces)
- Copy the page tree to another location, such as a small space or even just temporary space.
- After the copy is complete navigate to the 'new top level page'
- select the drop down page menu (... three dots)
- then select "Move",
- then move that page (which moves the enter tree) to the new location you want.
Summary: Copy the page tree to a small space, then after copy completes move the page to the desired location.