If a big difference remains between the tick times of the different shards, those who don't utilize the fast shard will be at a disadvantage on the slower shards.  

The fast shard could be used to create an attack force to send back to the slow shard.  When power creeps are implemented, the people that utilize the fast shard will be at a huge advantage if they decide to attack someone that has not used the fast shard.  They will level their power creep on the fast shard.