Hi, I have the same issue with my t3640. The CPU is i9 10900 (65 W). I tried to work with opened chassis but observed almost no difference. The temperature is often above 90 degrees C on CPU cores. It looks like the performance depends on the temperature. When it is cooler in the room, my tasks take less time to complete on that CPU. Still have no ideas how to overcome the issue.

1. replacing the cpu thermal grease with a "vertical carbon", diamond, or at least silver based. very thin layer.

2.putting the inlet fans, or at least removing all the obstacles, like extra HDD and DVD from the inlet air path. Small DELL machines are very poorly designed from this point of view.

I have found the culprit. The t3640 system with i9-10900 CPU is equipped with an entry-level heatsink because this CPU has the official TDP equal to 65 W. It was a joke from Intel. This CPU has actually two Power Limits: 65 W (PL1) and 224 W (PL2). The chipmaker only discloses the PL1 value, which is the processor's power draw at base clock speeds. The PL2 parameter, on the other hand, is significantly higher since it's the maximum power consumption that you can expect when the processor is pushed to the max with turbo engaged. Under full load the CPU works with maximum power during the specified tau, which is equal to 28 seconds for i9-10900. The installed heatsink is unable to dissipate the additional power. That causes overheating and throttling in my system. DELL has another heatsink for t3640 which can dissipate up to 125 W. Not the ideal but better than 65 W heatsink. I asked for the replacement but my request was refused.

The space in that case is very, very limited ( by what i can see looking at pics online ) and i don't think you can fit a 3rd party heatsink.

What about fitting inside a water cooled solution ?

I asked about the stock heatsink, not a 3rd party one. You can find it with this link .

Currently, this heatsink is installed in my machine.

I know. I was simply musing about dissipating more than the better stock one could (and so going to third party).

If it could be watercooled, it would handle the whole >200w of peak easily, I think

This is a new PC. I will most likely lose my warranty after installing a custom cooling system. But I agree that a water cooling solution works out the best for the 10900 CPU.

Turbo Boost is marketing not Guaranteed Speed.Precision 3640 Tower
Regulatory Model: D24M
Regulatory Type: D24M004

I added lots of fans and used splitters

https://www.amazon.com/gp/product/B07F8LV1BY

There are teeny tiny HDD fan headers but I have no idea what the part numbers for those are or for how they mount to the HDD caddy cages.  I improvised and mounted 92mm fan in the front hard drive cage moving the hdd caddy to the bottom.

https://www.amazon.com/gp/product/B07X24NTM3/

Getting Front fan and 120MM top fan helps but the real fix is difficult and the average user wont be able to get it to go in. Its not a 30 second water chicken install and its a VERY tight fit.  120MM top fan MUST GO IN before the heat-sink.   Heatsink 0T57JF Fan 0KTDJC

https://www.ebay.com/itm/274324703552

T57JF Heatsink

I

ts MUCH Easier to get front fan J9VTG Bracket  / 7M0F5 FAN in without breaking it by removing the hard drive caddy cage. moving hard drives to the bottom allows you to mount 92mm fan in the upper hard drive caddy bay.

There are also hard drive fan headers with tiny connectors but I don't know what those part numbers are.

TOP Fan is also 7M0F5  METAL BRACKET SAYS 13P1-4ZN0201

but I dont know what the DELL PPID number is.

120 mmTOP FAN Also 7FM05

Front 120MM FAN J9VTG Bracket

Is it possible to install a low-profile Noctua CPU cooler, such as NH-L12S within the Precision t3640?

Does anybody know? Thanks.

I agree. A closed water loop is a very good approach to spike heat output at turbo. The only problem with dell is that you can't put a radiator at the front panel without removing every single screw from there first. You have to move your drives to some improvised caddy not interfering with the radiator.