I am using newest (5.3.9) version of Cyberduck on OSX 10.12.3. After launch it uses about 180 MB of memory. The memory usage grows within 10 or so seconds to about 950 MB (no user input of connection to any server needed). Then over the next minute it grows up to 1.15 GB. The memory growth is not accompanied by any user input. This behaviour existed for at least one or two years. Is there a log I can attach? The Log Drawer is empty. Is there a Log I could attach?
I just compared 5.3.9 to latest 6.0 snapshot - I'm seeing pretty much the same huge memory usage as the original poster. I just launched Cyberduck (I have just about 50 bookmarks), opened & closed Prefs & Transfers windows several times, and am seeing memory use upwards of 800 megs, either version (slightly less in 6.0 snapshot than 5.3.9); see attached Activity Monitor screenshots. Also on 10.12.3.
As far as I am concerned the problem is not fixed. After the update was downloaded I have updated to a daily snapshot and the program behaves exactly the same way as before. Current version unmber is 23732