2019 iMac 4k External Monitor Lag

[shaunsheppard]

I purchased a new 2019 27" iMac (base model) earlier this week, and I purchased a BenQ EL287OU 28 Inch UHD 4k monitor to use as a second screen. The problem is, when I've connected the second monitor, at first the resolution high and everything was too small, so I adjusted that, but I'm finding that the screen is really laggy.

When I look at the display settings in System Preferences and the information provided on the monitor, I can see that it's 3840x2160@24hz. I've tried two different HDMI cables and a USB C to HDMI cable that I had spare, but none of these have made any difference. The monitor supports HDMI 2.0 and Display Port 1.4.

I've ordered a 4k USB C to DisplayPort cable which should arrive later today, which I'm hoping will solve the problem.

I thought I would post this first though, to ask 1) will the display port cable fix the problem? and 2) the iMac definitely supports and will work fine with a 4k external monitor?

EDIT: Sorry, this may be my fault, I've just noticed that the Novoo USB C Hub I have only supports 3840x2160@30hz anyway!! Which would explain to some extent why I'm not seeing more than 30hz.

 

[whosthis]

EDIT: Sorry, this may be my fault, I've just noticed that the Novoo USB C Hub I have only supports 3840x2160@30hz anyway!! Which would explain to some extent why I'm not seeing more than 30hz.

There is a good reason for this:

With USB-C (not Thunderbolt) the bandwidth limitation kicks in. You either have 4k@30Hz and USB 3 speed on the hub ports, or 4k@60Hz and USB 2 speed on the hub. Most hubs implement the first solution. There is also one hub around that has a switch so that you can make up your mind later.

With a real Thunderbold dock (same connector), you can have 4k@60Hz and speedy USB ports. But those hubs (or docks) are rather expensive - you might want to plug your screen right into the iMac.