Once you decide that you will have to lay people off, the time elapsed between making that decision and executing that decision should be as short as possible. If word leaks (which it will inevitably if you delay), then you will be faced with an additional set of issues. Employees will question managers and ask whether a layoff is coming. If the managers don’t know, they will look stupid. If the managers do know, they will either have to lie to their employees, contribute to the leak, or remain silent, which will create additional agitation.