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"?
Know how to save a wet phone?
It's not with a dryer and it's not with rice. CNET shows you the secret to saving your phone.