I've had an XPS 15 9530 for the past 3 years, and just upgraded to the 2017 XPS 15 9560 last week, with 32GB DDR4, 1TB SSD, 4K/UHD display. What should be a speed demon had some serious lagging issues under a variety of circumstances. Confident that the hardware was good, I did a LOT of online searching / sleuthing, and found three issues that all could be culprits in this kind of scenario. Spoiler alert: I found and fixed the problem with my new laptop, and now it runs beautifully.

1) Intel display driver
Dell (and other PC/laptop vendors) generally issue their own certified version of Intel drivers, and these may lag the latest versions that Intel offers themselves. I found that the Intel HD Graphics 630 driver currently provided by Dell is about 3 months older than the latest from Intel. Updating to the new one from Intel works perfectly well (as of 4/5/17), and seemed to fix some of the performance issues. Note: if you go into Device Manager and allow it to update the driver automatically, it will actually revert back to the older Dell approved driver. I preferred to stick with the newer Intel driver -- no negative side effects that I can discern at this point.

2) The XBox app in Windows 10
Open the XBox app, go to Settings and select the Game DVR tab. You'll see a switch to "Record game clips and screenshots using Game DVR". This is apparently NOT optimized for 4K/UHD screens, and is a major source of lag. Switch this off.

3) NVidia GeForce Experience app
By default, the "Share" option is turned ON. This is analogous to the "Record game clips and screenshots" option in the XBox app -- apparently designed to let you easily share scores, screenshots, etc. with other game players. For this, this was the primarily culprit: when I turned "Share" off, ALL of my mouse pointer lag issues disappeared - completely, and now the laptop positively flies as it should.

Disappointing that it's necessary to search to find such obscure solutions to issues that should not be happening at all. Happy, though, that the fix is easy. I am, once again, delighted with the new laptop, and continue to hang onto my old 9530 as a backup laptop. Both are excellent machines in general.

Jon