I would have to agree with the other posts that you may have a software issue with iTunes in general, but also from a hardware side, I find that 4GB of memory can still not be enough to run itunes smoothly on that MBP.
I have the very same model of Macbook Pro as you do with the 2.7Ghz Processor, but I always felt a little lag with the built-in 4gb mem. So I swapped it out for 8gb total and the whole system works much much faster. I honestly think that this model, and frankly any of the Sandy Bridge CPU's (2011 Macs) were released with too little memory! I also have a Mac Mini 2011, that came standard with 2gb and that is a complete joke. I upgraded it to 4gb, but now it really needs 8gb as well! I think the whole line really does. Just too much CPU power, and not enough Ram to really benetfit. The older macs seem to be fine with the less memory, but there is something about this line of intel chips that I really find from my personal use requires higher amounts of memory for lag free performance. Fortunately it is the cheapest fix you can do. I upgraded both my macs (they use the same memory modules) for like $42. online per mac. And that is high quality, reputible memory. Just shop around and 8gb is very easy to find. Just don't pay more than like $55. That is too much.
