November 13, 2012 from Anandtech:
By far the biggest issue with buying the 13-inch MacBook Pro with Retina Display is that you know, in about a year, it'll be updated with even better hardware. It's very clear to me that the 13-inch rMBP was built with Haswell in mind. Without enough room for a discrete GPU, a CPU with significantly faster processor graphics (~2x in the case of Haswell) will open up the 13 to even more customers
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Regarding the lag, true I was wrong. The problem is in the hardware, but not on the GPU:
Next-generation GPUs should do a better job of driving these ultra high resolution displays, but today it looks like our biggest bottlenecks are software and single threaded CPU performance. In every situation where UI frame rate drops significantly on the rMBP, the offending application usually ends up consuming 100% of a single CPU core
(...)
There's not much you can do here other than wait for faster hardware or buy the fastest CPU available on whatever system you're considering. Single threaded performance will scale linearly with CPU clock speed, so higher clocked CPUs should deliver smoother scrolling performance. Ultimately just scaling CPU clock is an inefficient way to solve the current UI frame rate issues. Future revisions of OS X will likely shift even more UI workload to the GPU, plus we'll see new microprocessor architectures that will perform better with these types of workloads as well.
Correct me if I'm wrong: actually, due to the OS, much of the work is demanded to the CPU that is simply not fast enough to achieve a good frame rate. Hence the problem is both in hardware (the CPU) and software (the OS). Maverick will shift much of the work to the GPU, but in any case you will need a capable GPU or the bottleneck will simply be shifted from CPU to GPU.
To sum up, Apple worked hard to solve the lag problem. New CPUs will provide much more power (and will be optimized to work with hi-res panel), Maverick will reduce the workload on CPU and will shift it to GPU, GPU that receives the most significative upgrade.