Not sure, might have figured out the problem. Testing now to see if it's fixed.
Attached image is downsized from original  1920x1080 render at default settings detail:5, AA:3, 1/4 sample, cache detail: 2, cache quality:2, max threads:64, buckets:256, Subdiv cache 1400.   23 min 19sec render time seems reasonable to me. Now I'm testing same size render at higher resolution to see what render time is.
I cancelled the original render after it ran 34+ hours  

  and was only 95% done.  looking at rendered node,  noticed detail and AA were 8, threads were 128,  buckets were 512MB, subdiv cache 2600 in the offending render node.  the tgd is one I reused from another project about 2 years back, and had several render nodes set up in it.  I may have screwed it up then and did not think of it when starting this render.   Hopefully will have proof tomorrow morning that issue is fixed.