Earlier quoted context omitted.
This isn't 2002 where single core performance had increased by 4x and average ram has quadrupled. Computers can now easily have a 10 year usable lifespan. Heck Nvidia provides drivers for your modern kernel for devices all the way back to 2007.
Single core perf has become more than 50% better on desktop since Sandy Bridge, and it should be even more than that in TDP constrained environments like laptops because of reduced power consumption, which is partly why we now have quad core / eight thread ultrabooks. The total performance of such a device would almost certainly be more than double an X220 in a slimmer package. Maybe 4x is your cut-off point for wher…
In actual use newer machines don't seem to come with much improvement in performance for a lot of tasks.
For me my limited supply of money is the primary constraint. I'll probably upgrade when my machine stops working.