I have Dell Latitude E6500 with P8600 CPU. When I run my Windows XP guest on VirtualBox OSE without virtualization VT-x option inactive it runs much faster than in the case I turn it on. It is really visible - with VT-x on I even see how screen flickers during the boot process.
I did Peacekeeper tests in Firefox and I got these results:
1) VT-x off, 3D support off - 1596
2) VT-x off, 3D support on - 1679
3) VT-x on, 3D support off - 900
4) VT-x on, 3D support on - 1314
Google Chrome behaves even more strange - with VT-x off it runs faster than Firefox while with VT-x on it runs *VERY* slow but looses no frames in graphic tests and it finally gets sth like 8800 points.
What could be wrong with my settings? I bought this machine to gain with using VT-x, not to decrease performace this way. ;/
I use 64-bit Karmic.
EDIT: I filed a bug report: