Because judgment on previous similar cases must be researched (jurisprudence) and then each charge refers to a law that has a specific range of sentence length.
Then, the judgement usually also accounts for different factors(good or bad) to determine the length of the sentence for each charge within the range determined by the laws.
There is also determining if the sentences are concurrent or not. And there is probably a lot of other stuff.
This is what I can think of from the top of my head.
The judge must also write the judgement in legalese, referencing all that was taken into account for the judgement.
And this isn't the only case that the judge has to complete. They have multiple going on at the same time.