I just completed my first project in MIR and I've found that the UI hang in Logic is unbearable. I have a rMBP 15" 2.6 w/16GB RAM as well as a late 2011 iMac 27" 3.4GHz w/16GB RAM.
The Retina MacBook Pro which has 512 SSD, is noticeably faster to load but when one attempts to install two or three plugin's be it MIR or other Vienna Suite Plugins with MIR the UI hang can get up to 30 seconds or more with fans spinning at full blast.
When the same feat is attempted in VEP5 both machines handle the load effortlessly. I have updated the OS on both machines as well as installed the latest version of code for all VSL products.
For now, I'm going to have to stay away from MIR as a native Logic Plugin until this issue gets resolved. VEP5 provides a MUCH more comfortable and flexible environment to work in with the exception of the convoluted way Audio Sends have to be routed. I hate bouncing tracks and seeing 1 core being maxed out into the red while 7 others sit idle!! :)
ccsr