My buddy Dave had a 2015 XPS 15 that would throttle just opening Chrome, swore up and down it was the CPU being old. Pulled the heatsink off one night and that paste was basically chalk, cracked and gray with a dry spot right over the die. Swapped it and dropped his load temps by almost 20C, thing felt like a new laptop for basic stuff. That said some of those Dell years also had bad fan curves and undersized coolers so paste alone won't always save you. Worth trying before you write the machine off though, worst case you're out 10 bucks and an hour.