Lightroom 6X slower to open a DNG than a PSD of the same file

Try resetting the Lightroom preferences (https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html )

Second if that doesn't help, try a new empty catalog file, import just the dng and run it from there. 

Third, if that doesn't help, create a new user on your machine and run Lightroom and see if that changes times.

I seem to remember (I am not a windows user) that having the indexing service turned on on windows can really slow down Lightroom startup times and other operations such as the import dialog, so something to try too. A quick google found this: https://www.online-tech-tips.com/computer-tips/simple-ways-to-increase-your-computers-performace-turn-off-indexing-on-your-local-drives/ No clue whether that would help here.

And that machine should do this much faster than mine! 14 cores vs 6 and my ssd is similar speed but the opening time should mostly be dominated by doing the demosaic and running the camera raw engine which is mostly cpu bound for this operation.

Do you by any chance have any virus scanning software running?

OK took your Grand Marnier file and imported it. I reset all the settings (there already was highlights, dehaze and shadows applied). Edit in Photoshop was approximately 2 seconds to open. Then I applied -10 highlights, +10 shadows and +3 deHaze and did an edit in Photoshop and it took exactly the same time of abouit 2 seconds. 

I am editing on a macBookPro 15" with a 2.9 GHz 6-Core Intel Core i9 processor and 32 GB memory so a fairly beefy machine but for files like this I would expect it to take no longer than a 2 to 5 seconds on most machines.

This inquiry may seem odd, and I may not be able to reply. But, as a diagnostic step

In LRC, accomplish an export of the DNG to a PSD, not an edit in, just an export, how slow?

So, for PSD , edit in PS, you are editing the original, no copy to be created. For DNG, that not being an option, either LRC or PS (I think LRC) creates a copy, a copy that has to be converted from RAW to raster.

But, that does look like way to much time. Mine goes a lot quicker.

Looked at whether applying any settings you mention and it makes no difference to how long it takes to open in Photoshop. About 2 seconds before the image shows up rendered in Photoshop from doing an Edit in with Photoshop already running. That's about what I would expect. Something is wrong in your setup it appears.

Just watched your video and realized you are not talking about opening the file in Lightroom but doing an edit in photoshop from Lightroom. What you are showing does not look normal at all. When I do an edit in Photoshop from a raw or dng file it opens in a few seconds, basically the same time as it takes to open a tiff or psd when doing an edit in Photoshop.

dj is right. importing a dng file and doing the initial calculations is way more intensive than a psd file. PSD files have a fully rendered preview in them that Lightroom can use straight away and there is no math needed to demosaic the data. When you read a dng raw file it has to be demosaiced, fast load data has to be generated and then the embedded edits applied before you can actually display. One thing to do to speed up import is to use embedded previews in the import panel and to not generate smart previews. 

File size is irrelevant here. Does the DNG have edits saved as metadata, including possibly local adjustments? This takes Lightroom time to crunch (which can be computationally intensive) and then display the edited DNG. The PSD file may not require any such intensive calculations.