Most likely the issue is not due to an assumed defective or damaged cpu. Replacing cpu w a new one will probably not solve your issue.  CPU almost never dies (except some newer 13/14th gen cpu that have premature degradation)

you can try run HWMonitor to find out cpu temp.  If the cpu was over heating it could slow down.

Next run Dell pre boot diagnostic test (PSA).  I presume cpu will pass.

Next run free user benchmark test to compare your cpu to peers.

You need to uncover the underlying culprit of slow cpu run frequency than pointing finger at a bad cpu (most likely not bad).

@redxps630​ Hi, thanks for your reply.

I have run the PSA and everything passed OK.

I also ran User Benchmark, results were:

UserBenchmarks: Game 19%, Desk 32%, Work 19%

CPU: Intel Core i9-12900K - 14.2%

GPU: Nvidia RTX 3080 - 106.3%

SSD: Samsung 860 Evo 2TB - 54.7%

SSD: Pc801 NVMe SK hynix 1TB - 151.8%

RAM: Unknown HMCG78MEBUA081N 2x16GB - 42.8%

MBD: Dell XPS 8950

I will run HWMonitor tomorrow (I have to go out now).

Thanks again.

Next time when you run user benchmark compare your cpu to others same cpu spec and see where you are on a bell curve. And you can run this test for as many times as you want to get an averaged performance test.

14.2%. seems way below average 

Average Bench: 117% (33rd of 1466)

Re: over the past week it's slowed to a crawl.  reset the BIOS to the default options

I think more likely your CMOs has corrupted settings which underclocks your cpu.  Check task manager>performance for cpu idle and run frequency 

Soft reset bios options within bios would not help.  You should probably try a hard reset (RTC reset) of CMOs.  

https://www.dell.com/support/manuals/en-us/xps-8950-desktop/xps-8950-service-manual/real-time-clock-rtc-reset?guid=guid-87b228f9-c91a-4409-88f0-7099a90421a9&lang=en-us#:~:text=You%20can%20initiate%20the%20RTC%20reset%20on,hold%20the%20power%20button%20for%2030%20seconds.

@redxps630​ Hi again, and thanks.

I followed the instructions on that page and it seemed to complete successfully, but I'm not sure - that page says that the process will reset the date and time - but it didn't. Everything in the BIOS seems as it was before (I took screenshots of all the relevant pages and nothing seems to have changed). Does that mean that the CMOS hards reset failed - despite the 'success' message?

There's no obvious change in speed either - I'm trying to run UserBenchmark again but the server is full :-(

UserBenchmark gives the same results as before. And I've tried setting minimum processor power in Windows to 100% but that makes no difference. Temperature is fine.

In Task Manager, System Idle Process is between 94-98%. CPU utilization is 0%, speed is 0.38GHz but Base Speed is 3.20GHz.

I also (on Dell tech support advice) installed Windows on a different 'know good HDD' but that was even worse.

Clearly, the problem is unrelated to Windows - even when booting the machine from cold the whole process is painfully slow. I watch Dell logo being drawn on the screen (*very* slowly), and the messages and screens that normally flash by before it shows the login screen are extremely sluggish.

So - assuming no hardware fault - what could cause the CPU to run so slowly?

Re: Does that mean that the CMOS hards reset failed?

try RTC reset again.  If settings are not reset then the RTC reset you did has not actually cleared cmos.

Re: speed is 0.38GHz

Try HWiNFO

Most throttling is either temperature related or power limit related. The HWiNFO Limit Reasons data is the best way to troubleshoot throttling problems.

get the Intel driver and support tool

It can find updates