My gig delivery times reflect how busy I am and how long it wil take to complete your order. If something is needed urgently, what you are doing is requiring me to work overtime and so there is an additional cost for that option.
If I am not busy then I deliver once it is completed.
Now, there are some misguided sellers who believe that having a big queue helps them in search rankings despite the fact that Fiverr says the opposite is true - delivering orders as quick as possible is favourable. This might explain some people acting in the way you mention but the majority are probably more in line with my rationale.