Recently I have upgraded to Dell Latitude E6510 with 4 cores / 8 threads processor, plenty of RAM, and a fast hard disk. Nevertheless, the interactive performance of Ubuntu becomes sloppy beyond any measure when a virtual machine or two start trashing the disks.
There seems to be known performance problems in Linux kernel, like Bug 12309. And full disk encryption makes things even worse. It appears that new FDE technology will give laptop users a chance to move the burden of encryption to hard drives.