t = db.distinct_speed1;

t.drop();
for (var i = 0; i < 10000; i++) {
    t.save({x: i % 10});
}

assert.eq(10, t.distinct("x").length, "A1");

function fast() {
    t.find().explain("executionStats").executionStats.executionTimeMillis;
}

function slow() {
    t.distinct("x");
}

for (i = 0; i < 3; i++) {
    print("it: " + Date.timeFunc(fast));
    print("di: " + Date.timeFunc(slow));
}

t.ensureIndex({x: 1});
t.distinct("x", {x: 5});