Turned out it was indeed a faulty motherboard and eventually Dell agreed to replace it, which resolved the issue. 
I've had this really weird issue since several months ago. For some reason the CPU is caped at 400MHz right after powering on the computer. And the CPU is not hot at all (only 37 degree celsius or so). As the result the computer is running extremely slow. I've tried everything I could: clean-installed windows 10 and 11 for three times, upgraded all drivers up-to-date. Tested all BIOS versions. All no help. Dell technician replaced the fan assembly and reapplied the thermal paste, no help either. The technician said it's highly likely a motherboard issue, and suggested me to contact Dell technical support on the already opened case to request MB replacement. However, technical support refused to replace the MB and instead, referred me to Software Department for some BS software diagnostic for additional $99 because "it's software problem, because all hardware diagnostic returned no issue". I don't understand what it has to do with software. I even ran the system with Linux and the CPU clock rate was capped at 400 MHz as well. I'm basically stuck and not sure what else to do other than paying $99 in addition to the 4 year warranty I bought when purchasing this laptop. It would be greatly appreciated if someone can shed some lights on this.  Really disappointed. Not just the product, but also the service Dell is providing.
Inspiron 15 5505 AMD Ryzen 5 4500U throttle at 400MHz, no overheating. clean 
installed OS