TB16 doesn't use dGPU, very high iGPU usage when doing anything

The TB16 can use the dGPU, no BIOS settings required.  It's true that the TB3 port is wired to the iGPU on that system, and that can't be changed, but the dGPU should still be available via NVIDIA Optimus.  The note you read about more expensive laptops is that they have a motherboard design that allows them to offer a BIOS option so that the user can choose which GPU directly controls the display outputs.  Giving the dGPU direct control of the output rather than using Optimus might be necessary for certain features that require direct control (VR, G-Sync, adaptive V-Sync, stereoscopic 3D, 5K resolution, and maybe others), but it's not required for general dGPU acceleration.  But the docks that can't use the dGPU in dual GPU systems are docks that use DisplayLink technology for video, like the D6000 -- but the TB16 doesn't use DisplayLink, because it can tap directly into GPU outputs.  And incidentally if you had a DisplayLink dock, you wouldn't be able to use the dGPU even with those more expensive laptops unless you completely disabled the iGPU system-wide (which they allow) because the DisplayLink issue relates to a limitation in Windows whereby only the "primary" GPU can be used for DisplayLink displays, and in a multi-GPU system, the Intel GPU is always primary.

When you ordered the TB16, did you confirm that it came with a 240W AC adapter?  The TB16 can also be ordered with a 180W adapter, and if you're using one of those, then the TB16 won't be providing enough power to run an XPS 15 system at optimal performance.  In that case, the system will throttle its performance to try to operate within the reduced power budget, and that can include not using the dGPU.  The system will NOT attempt to maintain maximum performance by drawing from the battery to make up the shortfall.  It will only do that under very heavy load conditions, but it will still keep its performance throttled at that point.  That might be why you're seeing normal performance when not connected to the dock and instead powered by either the battery or an AC adapter attached directly to the system.

Another option if you just received that TB16 would be to return it and get the newer WD19TB, which is the TB16's replacement.  I just suggested this to someone else and addressed some of the differences between the two docking station models in this thread.

Hi,

Thanks for your reply.

I looked at the WD19TB and it does seem nice, however I bought the TB16 second hand so I can' return it unfortunately. 

The TB16 did indeed came with a 180W adapter though as the laptop itself comes with a 130W adapter, would the dock really need 50W to operate? That seems rather high. I also didn't get a low wattage adapter warning from windows as many people have when using the WD15 for example, with an even lower wattage adapter. Do you know any way to confirm what the wattage output is to the laptop itself?

I have managed to enable dGPU acceleration by tweaking some nvidia control panel settings so my main issue is solved.

Thanks again

@cyborgium  the TB16 includes an "upstream" Thunderbolt 3 port that can provide I believe up to 60W to attached peripherals, so the AC adapter has to budget for that, since it obviously wouldn't be a good experience for the dock to say, "Well I can power your laptop properly if you don't attach certain peripherals, but if you do, then you'll have problems."  But that means that the dock will NOT simply provide more power to the system whenever you aren't using the upstream Thunderbolt port.  By comparison, the non-Thunderbolt WD15 model that doesn't have that upstream Thunderbolt port, and consequently it uses 130W and 180W adapters instead of 180W and 240W since the WD15 doesn't have to budget for that.  And the newer WD19TB only comes with a 180W adapter because although it has an upstream Thunderbolt port, I believe it now only offers 15W. But I'm quoting those figures from memory rather than reading the manuals at the moment.

The other factor with AC adapter sizing is that Dell didn't size the AC adapters for these docks precisely according to the power draw requirements.  Dell has been making 130W, 180W, and 240W AC adapters for various laptop models for years (like high-end Precision and Alienware systems), so even if they only needed slightly more than 180W coming from the wall in order to provide 130W to attached systems and run the TB16 and its peripherals, chances are they just decided it was simpler to just step up to the next AC adapter wattage level they were already manufacturing anyway rather than making something brand new like a 210W AC adapter or whatever.

But as for documentation, this page shows that all XPS 15 models require a 240W model, as do all other systems that are designed for a 130W power source.  It's true that the XPS 15 7590 isn't listed there, but the 7590 is designed for 130W just like all previous XPS 15 models, so it wouldn't be any different in this regard.  If you go into your BIOS Setup and check the Battery Information section, it will show the wattage of the detected power source.  If your TB16 is run by a 180W AC adapter, I believe it will only be receiving 90W from the dock.  The good news is that you can fix this by simply buying a 240W Dell AC adapter from somewhere, because again the TB16 just uses standard Dell AC adapters that have been manufactured for years anyway rather than some model proprietary to the TB16, so you shouldn't have too much difficulty finding one.  Then check the BIOS of your system again and confirm that it's now receiving 130W.  The downside is that from a quick check on Amazon, Dell 240W AC adapters seem a bit expensive, so having bought the wrong AC adapter size in the first place will increase your overall cost quite a bit here unless you can sell the 180W AC adapter.  But the alternatives are either a) running your system at lower than optimal performance whenever it's attached to the dock, or b) keeping the system's AC adapter directly connected to the system in addition to the TB16, which obviously isn't as convenient.

@cyborgium  that's very strange....  As for a high draw USB device, I don't know offhand of any USB peripherals would draw 60W or anything close except for other laptops, although I've never tried charging a laptop from the "upstream" port like that.  The newer iPad Pros that support USB PD can draw up to 30W from a source that supports 15V output, but I can't find that type of detail about that port, so unless you have an in-line USB-C multi-meter handy to show you exactly what voltage is being used and what amperage is being drawn, you'd be sort of in the dark.  But "conventional" USB devices like USB-powered external hard drives will only draw about 4.5W max because they're designed to be usable from typical USB-A ports, where 4.5A is the standard power output level for a USB 3.0 port.  Still, I wouldn't expect that to change the amount of power being sent to the laptop because the documentation doesn't mention that you can only use certain laptops if you avoid connecting certain types of peripherals, and that would be a pretty lame limitation anyway.  That's why I'm still amazed that the dock is presenting itself to the laptop as a 130W power source if the dock itself only has a 180W adapter, since that conflicts with the documentation I've seen and also wouldn't explain why a 240W adapter option would even exist.  The next step above 130W laptops is 180W laptops, but those require the TB18DC that can provide up to 210W to laptops because that dock connects to the system with two Thunderbolt connectors.  Just as a sanity check, you're absolutely certain that a) you have a TB16 rather than a WD19TB, the latter of which can provide 130W to a laptop even when running on a 180W source, b) that you have a 180W rather than 240W adapter, and c) that you do NOT also have the XPS 15's AC adapter directly connected to it?

@jphughan i figured no usb devices would ever draw that much, other then by charging something from it but I don't have anything that draws that much, I won't be able to check that unfortunately.

I just rechecked and I am 100% sure that it is the TB16 and that a 180W adapter is connected. According to the labels anyway though I doubt anyone would go to the trouble of switching the labels.. and the adapter that came with the dell is not connected either.

If what you say is all correct it really would not make any sense whatsoever.. I am confused myself.