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);