I'd like to know this answer too. Would like to know before I purchase!
I am using the brand nes iMac 5k with El Capitan. I am very disappointed with the performance. It ist laggy and slow, sometimes it's not possible to work .
What graphics card do you have?
What aspects of Lightroom, what functions, what tasks, are "laggy and slow"?
my machine has the AMD Radeon R9 M395 2048 MB.
LR sometimes becomes accidently extremely slower, so i have to restart the ap. Sometimes it crashes, so i have to reboot the system. Sometimes the switching from library to develop takes several seconds. Activating the gpu changes some issues, but brings some others, like switching through pictures and loading the 1:1 preview is slower then ...
27in. iMac 5k with 16 gig Ram OS X 10.11.2 running LR CC215.3 with absolutely no problems. CR2 files from Canon 5DRS (50 MP) are gorgeous. Highly recommend this system.
4 GHz Intel Core i7, AMD Radeon R9 2048 MD. Use Graphics Processor is ON (checked)
I have the up-to-date Version of LR CC:
Version von Lightroom: CC 2015.3 [ 1052427 ]
Lizenz: Creative Cloud
Betriebssystem: Mac OS 10
Version: 10.11 
Anzahl logischer Prozessoren: 4
Prozessorgeschwindigkeit: 3,3 GHz
Integrierter Speicher: 24.576,0 MB
Für Lightroom verfügbarer phys. Speicher: 24.576,0 MB
Von Lightroom verwendeter phys. Speicher: 1.180,7 MB (4,8%)
Von Lightroom verwendeter virtueller Speicher: 1.547,5 MB
Cache-Speichergröße: 114,2 MB
Maximale Anzahl Threads, die Camera Raw verwendet: 4
Camera Raw SIMD-Optimierung: SSE2,AVX,AVX2
Monitore/Anzeigegeräte: 1) 5120x2880
I do brushing and spot healing, also radiant filter is used quite often. I also did that on my old Windows system. There it wasn't smooth, but workable...