Creating linked sub-portfolios takes a lot of clicks. At least eight clicks for each, assuming the target portfolio is visible without scrolling in the dropdown in the dialog for Assign Sub-portfolio Link Target. Unfortunately the dropdown opens at the bottom of my portfolio tree. Defaulting to All Accounts might be more useful and intuitive.
One of the clicks is a seemingly unnecessary checkbox simply to open the dropdown. Why is that necessary?
How about enabling Alt-Drag to create a linked sub-portfolio, in the same way that Ctrl-Drag copies a sub-portfolio? That would be the fastest action. Alt-Drag-Drop currently seems to just do a Move, the same as Drag-Drop without the Alt key.
Another option could be to use the existing context menu Right-click->Copy followed by a new option for Paste as Linked Sub-Portfolio.