I do notice a lag on my brand new MacBook Pro 15" Retina when compared to a relatively new 27" iMac. From what I'm hearing, the lag is limited to retina-based Macs. That being said, it's a barely noticeable lag unless you're really looking for it.
I wouldn't say it's barely noticeable. After a reboot, there is no lag whatsoever. The lag always kicks in a few hours or a day or so later and stays that way until another reboot. When it starts lagging, it can get very bad, switching spaces can take several seconds (that's bad if you switch spaces every 5 seconds), Mission Control can take many seconds to load, and opening new tabs, dragging tabs around in Safari can freeze for like 10 seconds, it can even cause my computer (Late 2013 rMBP) to freeze completely. Rebooting isn't an option when you're in the middle of a 9 hour long project that requires constant FTP transfers, many hour long file transfers, saving massive Photoshop files and having dozens of windows open. I'm always doing something on my computer, I never have the chance to reboot.
Occasionally Safari will freeze and eventually the mouse will stop moving, and I'll have to do a hard reset. I've gotten kernel panics and complete freezes occasionally too, which, over the course of just 2 months, is very bad.
I want to downgrade to Mavericks but I don't know how long that will take, and I can't risk not being able to get work done because the restore fails, or I end up forgetting to transfer some new files, etc... And I have better things to do on my 1 day off a month. I have no more patience for computer problems, this is not 1999 when you spent a weekend setting up Windows 98 and installing drivers and getting the internet to work.
Modern operating systems should take into account that people rely on their computers like they rely on running water and electricity: it can't be my job to fix these things anymore because unfortunately the world expects you to have a working computer at all times. Imagine if your electric company installed some new transformer at your house, and from then on you would only have electricity every other day. And 3 months later they would still not have solved the problem.
