Unusable performance in LRC 10.0 MacOs Mojave 10.14.6 related to secondary monitor?

Hey there,

 I've just updated to LRC 10.0 and the performance has become completely unusable  when my secondary monitor is enabled, performance is smooth with just one displayed enabled within LR.   Specifically the issue is 30 seconds of spinning wheel hanging time every time  I move between photos and making simple slider adjustments. 

 Previous to this update, i've always worked with my dual monitor setup with smooth performance.  

  My machine specs are  : Mac Pro (Late 2013), 3 GHz 8-Core Intel Xeon E5, 64 GB 1866 MHz DDR3, AMD FirePro D700 6 GB.  Macos Mojave 10.14.6   The one of my monitors is 4k.

I'm guessing the problem is coming from a GPU driver issue, though turning off GPU display processing worsened performance.   

So far I've tried:

doubling my camera raw cache size, creating a new smaller library,  a complete uninstall and reinstall or LRC (including the prefs) rebuilding my smart previews