Hi Twelve and IntelEnthusias, thanks for your quick response!
I've tried CPUID-HWMonitor, Realtemp and HWiNFO64, all of them reported similar temperatures. HWiNFO64 showed only the core #3 got throttled as it hit 97c only few seconds after running the stress test (wPrime), while core 0 & 1 were sitting at 70c, and core 2 was low 90s.
I've been using this laptop for 1.5 years, I'm pretty sure it wasn't like this before as it could always finish wPrime 32M under 10s (I remember the max temperature was like low 80c for all the cores), and now it takes over 11s due to the throttling
I didn't modify anything since I bought it ... so maybe the thermal paste got burnt out?