Se types see things as done or not done. If they think something needs to be done and it's not, they just do it. They don't think about it too much or build it up in their heads. Stratiyevskaya's description of ESFps mentions that they often wonder why people make stuff out to be difficult when they're actually easy. They recognize that others make things harder than they have to be. Whether Se types realize it or not, the reason why it's easy for them to do things is because regardless of how much effort or time it will require to accomplish something, it can only be as difficult as they believe it to be.
Ni types see processes. They think about each step involved in doing the task, from trying to decide if they should do it to walking about from it when it's completed. They also have a good sense of when it needs to be started in order to finish it at the optimum time, so they often put things off until they actually need to be done. Although it comes naturally to them to juggle many approaching tasks and events in their minds, the thought processes involved in doing so makes them sort of build stuff up in their minds until it seems like a large, complex project. Seeing all of the steps play out at the same time in their minds makes them focus on the amount of effort that will be required, which makes it seem like more effort than it would to a Se type, who understands that there are different actions required to complete a project but just takes it one step at a time.