If you have lots of cores and ram you can tweak the env variable UNICORN_SIDEKIQS to a bigger number when you crank up ./bin/unicorn (I think it should work for ./bin/rails as well). Just how big depends on CPU cores, ram, database speed, and so on. You’ll need to experiment to find the best value. You can search for some more info here, but this is the basic idea.
And since you’re going to start from scratch again when you do the next import, it doesn’t matter that much right now. 