Photoshop Lag on External Monitor

[MikeArtworks]

Hi!

I currently use my MacBook Pro 13 inch with touch bar for everything I do (browsing, movie watching, designing and occasionally gaming).

I design in Photoshop CC (2017), and it works great when I'm not using my external monitor. Now the problem is that when I use the monitor and I have the Mac in clamshell mode, it starts being really choppy as soon as I use photoshop in full screen. Only when I scale the window down to about half of the screen, it starts running smoothly again.

I have a fully specced 13 inch 2016 (i7 3.3 ghz, 16gb ram) and I think it should be able to run it.. right?

Screen: LG 27 inch (usb C) screen 4k.

So in short:

When photoshop fills the screen on the external 4k monitor, it is really choppy, even when just adjusting a few shapes. Only way to get it smooth again is to make the window smaller, so scale the window down.

Any idea what I can do?

Thanks!

 

[MikeArtworks]

?

 

[Samuelsan2001]

How are you connecting to the monitor??

 

[MikeArtworks]

Thunderbolt 3..

I can have a 4k video running on the monitor and doing photoshop full screen on my laptop without lagging, but as soon as I full screen photoshop on the monitor, it becomes really choppy, even when the macbook (pro) is closed..