Remember that the rumours of lag issues in the first rMBP's were just that, rumours. They were, in the main, from sources such as paid Windows trolls, and zero-post members who didn't have the rMBP or hadn't used one.
Yes, there were issues with Safari and some other software when Retina first came out. These were quickly solved a month or two after the initial release of the software. It is unlikely this will be repeated as badly as the apps are now Retina-aware and the iMac 5k is just the same as the MBPs.
There were also some issues with rMBPs that had too much or wrongly applied thermal paste. These would overheat and therefore throttle badly. If you are unlucky enough to get a badly manufactured computer of any brand or type, you just return it or get it repaired within the warranty period. You will easily be able to tell, as you can compare temperatures with other users. Some users have reported temperatures up to 100C on a retina MBP. Even running Handbrake for an hour solid, or 3d games, or FCPX rendering, I can't get any part of my rMBP above 75C. If yours is running 100C then you should get it fixed. This would apply to any badly made iMac or MBP or anything else.
In short, there was no lag issue with the rMBP. There never was, and there hasn't been since. The very early reports to date indicate that the 5k iMac runs smoothly even without the GPU upgrade. I can't believe Apple would launch a machine with such an issue.
