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.
Some vendors already offer devices doing all the crypto stuff by themselves, on-the-fly, and supposedly with no performance impact whatsoever. I have heard about these drives a while ago, but just now came across this document from Seagate, which convinced me that their drive are likely to work in my laptop and Linux, or pretty much any laptop/OS with BIOS supporting password-protected hard drives.
I have just ordered ST9500421AS - Momentus 7200 FDE.2 500-GB Hard Drive for a bit over 62 euro before tax, we will see how it works out...