XPS 15 9550, CPU stuck at 0.78 GHz, solved

Issue

I updated to the BIOS to 1.12.0 the other day. Not sure if it's coincidental, but today I noticed the XPS performance was quite slow.

The CPU core speed on all four cores hovers around 798 MHz.

Windows Task Manager CPU utilization will not go beyond 34% even when stress testing the CPU with pass mark, userbench, etc. Games that run fine are now unplayable.

Update

It's interesting that Intel's Extreme Tuning Utility is reporting 100% CPU utilization, while the Windows Task Manager is peaking at 33-34% (which is where it's been peaking since this issue occurred).

Notes

All stress tests conducted with AC adapter plugged in and power plan set to Maximum.

Troubleshooting

Downgraded BIOS back to version 1.11.2: Performance unchanged.

Upgraded BIOS back to version 1.12.0: Performance unchanged.

Reset BIOS to factory defaults: Performance unchanged.

I ran Dell ePSA: Passed.

I ran Dell SupportAssist: Passed

I ran the Intel Processor Diagnostic Tool: Passed

--- IPDT64 - Revision: 4.1.4.36
--- IPDT64 - Start Time: 2019-11-27 2:04:26 AM

----------------------------------------------
-- Testing
----------------------------------------------
CPU 1 - Genuine Intel - Pass.
CPU 1 - BrandString - Pass.
CPU 1 - Cache - Pass.
CPU 1 - MMXSSE - Pass.
CPU 1 - IMC - Pass.
CPU 1 - Prime Number - Pass.
CPU 1 - Floating Point - Pass.
CPU 1 - Math - Pass.
CPU 1 - GPUStressW - Pass.
CPU 1 - CPULoad - Pass.
CPU 1 - CPUFreq - Pass.

IPDT64 Passed
--- IPDT64 - Revision: 4.1.4.36
--- IPDT64 - End Time: 2019-11-27 2:09:06 AM

----------------------------------------------
PASS

Specs

HWiNFO logs here https://pastebin.com/raw/By8YDcnu

I'm not sure how to proceed next other than chucking the laptop in the garbage.