When i compare performance with version 1.6.1 of the driver i get the following results for 1000 finds.

Version 1.6.1
0.109000 0.062000 0.171000 ( 0.232013)

Version 1.9.2
0.062000 0.000000 0.062000 ( 7.334419)

Used the following code

require 'benchmark'
require 'mongo'
mongo_client = Mongo::Connection.new('localhost', 27017)
db = mongo_client.db('sample-db')
col = db.collection('requests')

time = Benchmark.measure do
(1..1000).each { |i| col.find({:key => i}).next_document }
end
puts time