Collins and Duffy (2001) report that in practice, running time for the parse tree kernel is more close to linear (O (|N 1 |+|N 2 |), rather than O(|N 1 |*|N 2 | ).
using the PC with Intel P4 3.0G CPU and 2G RAM, our system only takes about 110 minutes and 30 minutes to do training on the ACE 2003 (~77k training instances) and 2004 (~33k training instances) data, respectively.