It all depends on where in the job you decide to cancel it. If there is a byte count, you can cancel very quickly. If the job is loading media, scanning a tape looking for space to append to, etc it can take a long time until that process is complete before the job can cancel. Basically, if data is being written you should be able to cancel quick, otherwise there are a lot of issues which can cause a slow cancel while it waits for some other process to compelete.