Below is a benchmark result.
My environment is Ubuntu 12.04 on Parallels(virtual machine) on Mac OSX(Because I don't have any Linux machines). So there may be a bit of overhead although C implementation score is even 1.15 sec on native Mac environment. CPU is 1.7GHz Core i5 and 1 core is assigned to Parallels.
By the way Ubuntu 12.04 is the easiest platform to try HHVM currently.Taiju Muto 2013.3.15