Since I am almoust done with my remake of all the new tilesets for my issgard Island , I am at the major point of deciding if or not convert my cities into closed ones similar to bethesda or instead leve them open like morrowind .

The obvious drawbacks could be that since my cities are at least three times the largest one of vanilla that they might be 1 difficult to encircle in walls by al sides and second that they add lot of new things that might slow down perhaps on some smaller machines.

Is there a way to test performances ? I have a good pc and have no signs of slow downs but others might be less lucky.

The other issue I see is for eventually dragonriders ( I think there are some mods that allow that ) that could spy over the borders of a cit or in theory be able to overcome walls.
Finally my cities are all but flat and develop usually more vertically than vanilla so parts might be accessible from climbing mountain sides or the like ....

This is why I need to carefully plan what to do if close them or make them open.

Skyrim Performance Monitor. That's probably your best bet for checking performance. If your cities are 3x the size of Whiterun you definitely need to check depending on how much clutter you're including because it may not make the slightest difference whether they're open or closed at that size.

The obvious preference is to open them up. The only reason that Open Cities Skyrim seems to be having any trouble at all is because something in Patch 1.9 strongly disagrees with the cities being transferred out of their original locations. When I last tested with the performance monitor, there was no difference at all in frame rates. It was just a matter of the game engine deciding it didn't want to do what was being asked of it.