Performance of count() on range check queries is worse than it should be

Using the test script below this script reports about 50ms of time taken for the count query in the script. This is way too slow for a query that the query optimizer should resolve to a direct bsearch on the index, or so I assume.

use test

db.values.drop();
db.values.ensureIndex(

);

for (var i = 1; i <= 100000; i++) db.values.save(

);

var t1 = new Date().getTime();
db.values.count({value:{$gt:5000}});
var t2 = new Date().getTime();

(t2 - t1);