How can I scale a 4K external monitor to have a comfortable amount of space I'm using a 13inch touch-bar MacBook pro with a Samsung U28E590 4k monitor.

I'm very happy with the resolution on the MacBook's screen and everything is a comfortable size. However on the monitor the "default for display" (which says "looks like 1920x1080) makes everything huge and sacrifices the amount of space on the screen.

If I go into display preferences and choose the middle option (3008x1692) this is a good size, but it has a noticeably bad effect on performance - window switching, resizing etc is jerkier and slower.

The setting I currently have it on is "more space" (3180x2160) which doesn't suffer the performance issues but makes everything tiny. I have to zoom in on any applications I use in this monitor (mostly terminal and text editors).

Is there a nicer solution to having a comfortable resolution on a screen like this without sacrificing performance?
Using Retina mode on a 4k display is a performance demanding task - especially for that 13" MacBook Pro. When you choose "odd" resolutions like that, the operating system will have to render in really high resolutions and down-sample to get an output suitable for your monitor.

What you could do instead was to install SwitchResX, Display Menu or similar apps that allows you to configure the monitor to your liking. I would recommend trying to disable Retina mode and simply use a high resolution with the monitor (not 3840x2160, but something smaller). 

If you disable Retina mode, the performance demands are considerably less.