If the Windows install is more than a couple of years old, a clean install may do the job. Beyond that, it's an older system with a CPU that wasn't all that capable when the system was new - -and 8G is quickly going the way of that 4G used to be -- entry level.  Unfortunately, the RAM cannot be upgraded (at least not without a replacement system board).  
I have an XPS 13 9343 with an i5-5200U CPU @ 2.2GHz and 8GB of ram running Windows 10 Pro. The performance has slowly gotten worse over the last year or so. It has gotten to a point where any web browser (Firefox, Chrome or Edge) won't fully load web pages. The CPU hits 100% usage and the usage memory approaches 100% and it just hits a standstill. Is it time to bite the bullet and buy a new laptop or is there something I can try to speed it up?
XPS 13 Slow processing