There are many reports about speed issues with 2015.6, specially with Nvidia GTX cards.
I think there will be a fix but I have no clue when and Adobe does not tell.
You may want to roll back if it bugs you too much. Here's hoew: How do I roll back to Lightroom 2015.1.1 or Lightroom 6.1.1? | The Lightroom Queen
Simply replace 6.1.1 with 6.5.1 in the guide.
I disabled my GPU AMD Radeon and it started allowing me to edit my photos again. ADOBE get this fixed as LR CC is buggy as right now.
Well glad it's not just me. Hope Adobe fixes this soon. LR is so slow right now and I don't want to jump ship.