Question my dell g5 5500 overheat and fps drops

what do u mean with Thermal throttling ?
It means the CPU can't maintain base clock speeds. So if it's struggling to maintain 2.6GHz, it's thermal throttling.

Looking at a review for your laptop, I did find it gets up to 104C while playing The Witcher 3. It's likely the laptop starts at a really fast speed giving the perception of what you should be getting, but once it hits that thermal barrier, clock speeds start dropping to keep it in check. But unless the CPU is dropping below 2.6GHz even after cleaning the vents and fans, this is basically normal behavior.
 
It means the CPU can't maintain base clock speeds. So if it's struggling to maintain 2.6GHz, it's thermal throttling.

Looking at a review for your laptop, I did find it gets up to 104C while playing The Witcher 3. It's likely the laptop starts at a really fast speed giving the perception of what you should be getting, but once it hits that thermal barrier, clock speeds start dropping to keep it in check. But unless the CPU is dropping below 2.6GHz even after cleaning the vents and fans, this is basically normal behavior.
thank u sm