Go to the Preferences dialog, Performance tab.
Try turning off the GPU optimization and see what happens.
I'm working with D810 RAW files just fine.
I suspect it to be the 8 cores Ryzen. LR currently seems to have issues with more than 4 cores.
There are some threads that discuss this and even show up a temporary fix until this is fixed.
Use the forum search ..
Please check is there any pending OS update left, also update the gpu drivers from the manufacturer website.
Please tell us exactly what actions in Lightroom are slow.