Slow and laggy laptop

Hey Dell forums,

After looking all over the internet for answers with no luck what so ever, I've been forced to write in a forum like this - I really hope that you can help me.

The past few weeks or so I've been having a lot of problems with my almost brand new Dell laptop. It's a Dell XPS 13 (2015), with an Intel Quad-core I7-5500U CPU and 8 gigs of RAM. Lately it has been performing really laggy and slow. I have another laptop I use for work with a worse processor and RAM than my own, and it is performing a lot more snappy. 

Just some examples: In chrome pages takes AGES to load, and when the page is loaded it feels laggy and unresponding at times. Small online games have massive frame drops and lags like crazy. 

When looking at the performance tab in task manager, the CPU seems to be locked at 0,77 GHz, even though it should be able to run at 2,4GHz. What can cause this? What can I do to boost it and make the computer responsive and actually usuable? Or does it have nothing to do with the CPU? 

I've tried updating my BIOS, as I've recently upgraded to Windows 10, and I've tried to delete all unnecessary programs and files, but nothing seems to work. 

I really hope you can help me with this, as I'm very frustrated that such an expensive purchase doesn't perfrom how it should. 

Best regards,
Peter