Many users going from 2010-2012 iMacs to Retina iMacs reported slow performance, especially in our Agenda module. The Agenda module SQL's an appointment table and fills repeating fields accordingly, after having emptied them via lookup. We did some timing via UTC and saw that the scripts are finishing their job in max 0.6 seconds, whereas the perceived update time was more along 2 seconds.
IMPROVEMENT (not solution):
We saw performance going from unacceptable to acceptable by touching the following system preferences:
1) General: disable "Use LCD font smoothing when available", "show scrollbars : always"
2) Desktop and screen saver - Desktop - use Solid colors
3) Desktop and screen saver - Screen saver - use message, Start after: never.
4) Screen - color - Adobe RGB(1998) instead of the default one: will go from 10 bits per pixel to 8 bits per pixel.
5) Energy saver: Turn display off after :never
5) Energy saver: disable put hard disks to sleep