i'm totally not surprised to hear about your lag issues. i've been saying this all over the forum, that despite Apple touting the A6X as "twice as fast", the A6X still isn't even sufficient enough to utilize the Retina display's full capability. i'm not talking about web browsing and gchatting, i'm talking about what you said above...playing a graphics intensive game such as that or Metal Storm. powering the Retina ends up being the bottleneck and the achilles heel of the iPad 3 and iPad 4. neither the A5X nor the A6X truly is sufficient. the iPad 2 with A5 has superior gaming performance over the generations of iPads that Apple touts as perfect for gaming (how ironic)
i've used both the iPad 3 and iPad 4 extensively, and the performance improvements currently are nearly negligible and limited to minute basic operations such as opening a folder of apps and web page scrolling. my iPad 4 loaded web pages faster by a second at most, compared to the iPad 3.
i bounced around from a brand new iPad 3, apple allowed a return and i got the iPad 4. saw how insignificant the differences were after playing with it for 2 weeks, returned it and got an iPad Mini. this device is great, but there is massive light bleed and it's impossible to do an exchange as no Apple store within a 50+ mile radius of me that has the 64GB LTE-Verizon in stock. i decided that i will be returned it tomorrow and picking up an iPad 3 64GB LTE-Verizon that i ordered online last week. it came to $679 - just $20 more than the Mini. i think i will use that until the iPad 5 comes out.
overall i am thoroughly disappointed in the iPad 4 w/ A6X just as much as you do. those performance tests show that the A6X is a beast compared to the A5X, but real world use just doesn't reflect that and most likely won't reflect that. the A6X will end up being a blip on the timeline as Apple realizes the Retina still needs something better and rolls out the A7X aggressively (or whatever it's called next)