Some of the time required is built in to the process. For example, the court reporter and court clerk are given time to prepare transcripts and gather and transmit the record to the appeals court. There are also fixed time periods for filing briefs. Once an appeal is ready, it is assigned to a department for consideration as soon as possible. After consideration, the department issues most decisions within 90 days.
Some decisions take longer.
That is sometimes true in complex cases or when the three judges of the department cannot are not unanimous in their opinion. In such a case, the time to decision after oral argument can be as long as a year. The court schedules oral argument only if one of the parties timely requests it, and only if the court agrees that oral argument might be helpful in deciding the case. However, the court generally favors oral argument if there is any doubt about its usefulness. So requests for oral argument are generally granted.