Why Did It Take So Long?

Justice, even in this vale of tears, can be an exceedingly slow and uncertain process. The sentence for Mr Bates of Downton Abbey came soon after the jury found him guilty. Why can't we do the same?

Surely the judge has heard the same evidence as the jury. Why shouldn't he or she sentence the defendant immediately? Mitigating factors are always brought up during the trial, are they not? So what is the delay for? And why should we commoners have to put up with harassing the poor convicted criminal for 20 years?

And isn't a delay of 20 years a form of "cruel and unusual punishment?" Or so it seems to me. Where have I gone wrong?