I run office and stock trading applications on an i5 retina system with the base GPU. No games or video/photo editing.
With my use, all 3 displays are very smooth. No lag at all. The other monitors are cheap Korean 27" 2560x1440 displays so I needed to buy the dual-link DVI adapters. I tried various other connections but this was the only one that game me the full resolution.
I had a PC based system and when that died, I moved to a 2012 i7 Mac mini. Unfortunately that wasn't up to driving 3 displays very well and also ran very hot.
The iMac runs in the 60s and is perfect for what I need.
I got this with a 3TB fusion drive and 16gb memory from the refurbished store, which seemed like a good deal, and should last me a while.