There does appear to be a serious regression somewhere when it comes to the ability of VirtualBox for building a vanilla Linux kernel. Building the vanilla Linux 3.1 kernel took significantly longer under VirtualBox 4.2 RC1 than when using the current stable release (v4.1.20).
There are some small performance improvements in some of the tests, such as Smallpt, but that test for instance still has significant overhead compared to the raw/bare-metal performance on the multi-core Intel Linux system.
If you were hoping for some performance breakthroughs with Oracle VirtualBox 4.2, it sadly doesn't look that way, at least when using an Intel Linux system and an Ubuntu 12.04 LTS guest. A few tests improved by small amounts with VirtualBox 4.2 RC1 over the 4.1.20 stable release, but there weren't any clear trends and there were also some performance regressions (namely with the build-linux-kernel test profile). More benchmarks of VirtualBox will be conducted when the 4.2 release is officially available.