No offence but I'm convinced people who claim no lag/frame rate drop either don't push their machines very hard or maybe do not know the difference.
If the older machine was slower in CPU and GPU and no one had lag issues.
It's more likely that the new GPU needs better drivers, or there is an issue someplace else. the computer is NOT underpowered.
I used two run my older 13in model with its display and an external 1920x1200 LCD. If it was an issue of being "under powered" then trying to use that external LCD would have been impossible. I would have many Chrome Tabs, and Netflix, and java chartrooms, Word, Excel, IRC and Xcode at once.