Earlier this year I received a brandnew Dell Latitude 7490 as my notebook for work and since then I am really content concerning the overall performance. However, the first time I wanted to use the laptop while working from home, it caused a huge disappointment. After connecting it via HDMI to my UHD external monitor, I noticed a drastic lag or stutter while working. Knowing that the monitor works perfectly together with my desktop PC and taking into account that the new laptop should have more than enough computing power for such a high resolution, I began to search for a solution.
Soon I found out that the HDMI connection is causing the lag. Even though HDMI 2.0 (and later) is a standard that was already established a couple of years ago, Dell only implemented a HDMI 1.4 port for its new business notebook. This outdated version of HDMI is limited to 30 hz when using a 4k or UHD resolution. However, if you want to work without too much of a noticeable lag 60 hz or more are necessary. In the end I can only give you the advice, to pay attention to the HDMI specification whenever you buy a computer.
Fortunately, in this case there is a solution to the problem: The laptop is also equipped with a Thunderbolt / USB-C port for which you can buy an adapter to connect it to an external monitor using the HDMI connection. I chose the following adapter which solved the issue: https://www.amazon.de/gp/product/B075V68NVR/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1