FAQ says:

How is the speed of a coin calculated?
Every transaction has a time when it was first seen in the network (timereceived) and a time when it was included in a block (blocktime). The difference between those two timestamps is calculated as a laziness factor. The higher it is (the more time a payment needed to be included in a block) the slower is the coin. The least lazy coin in a race wins the biggest part of the pot. If there are two or more coins with the same laziness in the same race, the alphabetical order of the transaction id decides which coin is faster.