What food processor or tools can help save time? I made 15 liters (4 gallons) of sauerkraut, but it took forever to finely chop all the cabbages by hand and beat it to get the juice to come out.
As mentioned a mandolin might help you here, but if you don't already have one and you are trying to keep to a minimalist budget (& kitchen) I'd stick to your sharpest chef's knife, and carefully consider your surface prep area.
For starters, chopping that much cabbage is going to take some time, but it's rather efficient when you consider time spent by the number of liters of sauerkraut made. Keep that in mind. Making 15L probably takes you only 2-3x more time than making 1L.
Next up is a sharp knife (8" minimum). Learn how to both sharpen and hone your knife, and the job will go must faster and the results will be much cleaner. For a job like this its helpful to run your knife back over your honing steel after every cabbage or so (5-10 strokes per side - just enough to keep the edge true)
Finally - have a large, clutter free prep area. Your chopping block/cutting board should be as large as possible, but don't attempt to keep chopping until you are working off an ever-shrinking area of your board. Instead, chop ~1/2 cabbage, then dump it all into your food bin/bowl. Repeat. This is how its done in a commercial kitchen when you have to chop 20lbs of onions or whatever.
As always, with lots of practice comes both speed and precision. Good luck!