So, the newest portal block for going to the outer towns of the end, "endgateway", is extremely laggy. It has a great texture and would love to use it for portals... however, it drops my computer from 1200 FPS to around 50 FPS so I can only image what it would be like for someone with a lower end graphics card.
http://prntscr.com/ahsuay - no portal block
http://prntscr.com/ahsujp - portal block