Look, this has been discussed and it is reported that any issues are being addressed and will make it into future Lightroom versions.
With Lr6 rumoured (wasn't March 9th) having GPU usage you can make your own assumptions.
I've responded to where you commented on another thread. I use a base i5 Retina iMac with the base GPU. Other Retina iMac users with full upgrades report exactly the same though, so not hardware dependent.
There are not severe performance issues at all. There is a mild lag with some tools such as Crop, the vast majority of time LR is fine on the iMac retina. You don't have to use LR at quarter screen.
The issue is LR and the way it is coded. The iMac retina works great with other photography software such as Capture One Pro and Affinity and Pixelmator. which supports the view it is LR, and only some bits of LR, that are lagging.
If you really can't live with that then simply start LR up in low resolution, it works perfectly then, you don;t have to use it at quarter screen or anything like that, until LR 6 comes along with hopefully the rumoured performance improvement.
The 5K has meant a great improvement in my workflow across not just photography but a whole host of productivity software because of the useable screen real estate and clarity of everything, especially text.
Very helpful response. I don't like the scale of 4K on a 27" screen, and 24" screens are now just too small. Glad to hear 5K is working for you.
Wish I had a crystal ball last year to know the 5K iMac was on it's way. Bought the Mac Pro thinking 4k was a ways off for the iMac. Now here I sit, waiting for a 5K monitor to drop below $2,000.