1. Go back to factory conditions (system recovery) via the Dell prescribed method (most likely: starting with ctrl-F11 early during boot).
2. If still issues, call them for warranty. If no issues, it's solved.
By the way, what is "too slow"?
iPhone 8: Everything we know so far
This is all the iPhone 8 reports and rumors in one place. From a 5.8-inch OLED display, reports of wireless charging and even a 3D scanner for facial recognition, it's all here.