Onset occurs when the insulin reaches the bloodstream and begins lowering blood glucose. Insulins with long onset (2 to 4 hours) are typically the long-acting insulins, or those that have long duration. Those insulins with the shortest onset times (30 minutes) belong to the fast-acting category, or those with relatively short duration. The intermediate-acting insulins have a 1-2 hour onset with 8-12 hours of duration. 
Among the fast-acting insulins, those based on human insulin, also called Analog insulins, such as Humalog, Novolog/NovoRapid and Apidra, have the fastest onsets. In general, any insulin that has no type of suspension will have a quick onset.
The long-acting analog insulins Lantus and Levemir don't have the "traditional" types of suspension but they are prolonged by other means. Lantus doesn't form crystals until it's under the skin; the crystals are slow to absorb. Levemir uses a binding to albumin in the bloodstream to prolong its action.
Working with Onset
A predictable onset is a necessary part of working with overlap. Insulins with short onset frequently peak in less than 4 hours after injection. Food can be given to extend onset and peak response.