In my opinion, the key issues include aspects of all of these, but I think that the major driving force is partisan.
The Ninth Circuit is one of the most liberal circuit courts in the Federal system. This is fine (for the most part) with people in the coastal regions that it includes. However, the people in the inland areas of Washington and Oregon (and all of Idaho, Montana, Nevada and Arizona) do not really care to be in a circuit that is dominated by liberals. So that is somewhat of a regional issue, but it is only an issue because of partisanship.
Although the partisanship is the main issue, there is a lot of talk of efficiency because the circuit has so many cases. This serves as sort of a cover so that people don't have to talk in nakedly partisan terms.