Maybe your performance issues are caused by plugins? I have no problems using IntelliJ on a base model 12" MacBook (1.1GHz Core M, 8GB RAM, 256GB PCIe SSD). I don't use Symfony2, though.
Side note: I am of the opinion that no matter you're doing, SSDs are a must. If you don't already use one as your primary drive, please upgrade. Once you go SSD, any machine with platters will feel broken. This goes double for laptops, which often contain slower 7200RPM and 5400RPM drives.