It's solved. For anyone else who might be in a similar boat, it turns out the processor was throttling, although I still don't know why. I simply drained the battery completely by playing videos on a loop and manually attempting to reboot it multiple times by pressing the start button to drain the remaining charge. Others have also suggested opening the back and simply disconnecting and reconnecting the battery to achieve similar results. Everything's back to normal now.
My 6 year old XPS 15 9550 with decent hardware specs has been underperforming in serious ways, going down gradually. I don't game or do any intensive tasks on it, but even routine browsing has become slow (across different browsers). Please find attached a screenshot of its performance test results from UserBenchmark. Both the CPU and the Graphics Card seem to be performing terribly. Am not sure if the NVidia graphics card is even working. [IMAGE] Some of the issues I am facing: General tardiness all over, irrespective of the program used. Can't play any 4K videos on it anymore (keeps stuttering). It takes forever to load some webpages. It stutters even on the Ookla Speedtest webpage, and surprisingly, gets half the internet speed on LAN when compared to 5GHz WiFi (yes, WiFi is faster). I keep the system BIOS and drivers updated, and have checked and corrected registry corruptions and removed potential malware. Also upgraded the power adapter and cable. Haven't opened and cleaned the laptop inside yet as I am waiting for a replacement torx screwdriver to be delivered, but have cleaned and vacuumed it from the outside, and haven't experienced any issues with noisy fans either. HWiNFO shows average temperatures and no throttling. I am not sure what is causing it to underperform. Any suggestions to solve this will be much appreciated. Thanks! 
XPS 15 9550 Terrible performance