It was relatively easy to get Ardour 2.X “bogged down” when working on a large session with many regions and lots of edits. This was normally the result of the undo/redo history becoming too big, which wasn’t hard because in Ardour 2.X, it stored too much information.
In Ardour 3.0, we have completely redesigned the undo/redo system to store only the data actually necessary to undo or redo actions. It consumes less space in memory, less space on disk and is faster (in some cases much faster) when actually used to undo or redo. We have seen (big) operations that took 40+ seconds to undo with Ardour 2.X that now take less than a second to revert.