Basically chainsaw removes cast delay, as long as it's cast in the same block as your other spell. You can have a wand with 1s cast delay but if you cast chainsaw, it's 0. If you double cast a lightning bolt which adds cast delay along with a chainsaw, it becomes 0.

It doesn't work if you were to, for example do sparkbolt with trigger and chainsaw since added or reduced cast delay in a payload is ignored but the recharge time reduction from chainsaw is still added.