Posting you crash reports details would be helpful
The fix that saved my day! I just purchased a similar Dell Area 51 R5 and experienced exactly the same issue. Just one click on 'disable' in the BIOS solved what would have been a monumental problem for me. Thanks for recovering that golden post.
Definitely an item to put in Adobe's knowledge system for all other Dell alienware photoshop and lightroom users.
Something similar has been reported several times, with X299 chipsets and certain CPUs (can't recall which). A BIOS update to the motherboard should fix it.
Other threads should come up in a search.
You are absolutely right, based on your feedback I have found that for the Dell Area 51 R5 there is a urgent update available that fixes the issue:
I checked my BIOS version and it was on 1.1.9, so downloaded the 1.1.10 BIOS update and installed it. Then I enabled 'Speedstep' option again to test Lightroom and now for me it works fine. I read that disenabling Speedstep in the long run will have negative impact the overall lifespan of your processor. As with BIOS updates, some care is required, so best to check the bios version and read the BIOS update approach on the Dell Alienware support site.