If you are one of the lucky people to have a new Dell XPS 15 (9550) aka the one with the new InfinityEdge display you will want to make sure to grab the new 01.01.15 bios released earlier this week.
The update can be found by using the Dell update app on some PCs or by using Dell's Driver Update page found here on the Dell site. This update is rather significant due to the changelog and what it fixes and addresses.
As can be seen above, the update improves audio performance, NVMe battery life, i7 CPU, system boot time, and sleep resume time. Those are some substantial fixes and users are strongly encouraged to grab the update for the best performance.
Unfortunately, no such update exists yet for the Dell XPS 13 (9350). However, that device has had updates in the past as well and is expected to get more as needed.
Windows Central is currently in the process of reviewing both the XPS 13 and XPS 15 and will have more in the coming weeks.