From what I've seen, the bottleneck that creates long voting wait times is not filling out the ballot itself, but systemic issues which limit the number of people who can cast ballots at any given moment. Ideally we want the rate of people exiting the polling place to be comparable to the rate of people entering the polling place. It's much like a tollbooth, where if twenty cars arrive per minute, we want twenty cars departing per minute, however long it takes each car to be processed. And like a tollbooth, if there are twenty arrivals per minute but only five departures, traffic will pile up into a huge jam.
Decreasing the amount of time that people spend completing the ballot itself will decrease the time they personally spend filling out the ballot, and that may increase the rate of departure. But there is there's a functional limit on how much improvement can be gained that way. It's unlikely we could reduce the average ballot-casting time even by a third, since part of that time involves being processed by poll workers, only a portion of voters will want to use that 'party-line' option, and certain things on ballot — ranked or multiple choice contests, ballot initiatives, state and local ordinances — do not lend themselves to simple party-line distinctions.
A far more effective system for reducing backlogs — and one that is often used at tollbooths, incidentally — is to open more stations. Doubling the number of voting stations within a given polling place will on average double the rate of departure; tripling the number will triple the rate of departure. There's a cost/benefit analysis here: at a certain point average wait time is low enough that adding more stations becomes an unnecessary expense. But state and local governments have more than enough data to predict through-flow rates at polling places and provide sufficient equipment, so there's no particular reason that any polling place should not be equipped so that they never have than half an hour's wait time.
Of course, it's fairly self-evident at this point that bottlenecking is an intentional system of voter suppressions. We only hear reports about three hour voting wait times in low-income, minority-heavy urban districts. Restricting the through-flow in such polling places is another system meant to restrict the minority vote by imposing obstacles on the voting process. It's shameful.