One of these is around 10x faster than the other two

set-seq-splitting-efficiency.js

// filtered is an Immutable.Seq (lazy sequence) with 307 entries
let withinCountry = filtered.filter(p => p.get('locations').some(isSameCountry(yourLocation)));

let outsideCountry1 = filtered.filter(p => !withinCountry.includes(p));
let outsideCountry2 = filtered.filter(p => !p.get('locations').some(isSameCountry(yourLocation)));
let outsideCountry3 = filtered.toSet().subtract(withinCountry);

return withinCountry.sort(someWayOfSorting)
  .concat(outsideCountryN.sort(someOtherWayOfSorting))
  .take(10);