Gavin was testing his proposed changes, that include increasing the max block size from 1 MB to 20 MB.  He noticed that the nodes  were taking too long (hundreds of milliseconds) to validate a block, even when the node had seen all transactions before and thus did not need to validate them again.  That wasted time impacted the time needed to propagate a block from node to node.  He tweaked and hacked the code until he got that part of the code to run a lot faster, saving some memory too.  That improvement reduced the estimated 1MB-block propagation time, in that easy case, from 600 ms to 10 ms.