Several performance issues have been identified in mongos. SERVER-7231 addresses one issue with logging in ParallelSortClusterCursor. There is much more room for optimization, though.

Baseline

In a simple test which floods mongos with a valid findOne() query, mongos was initially handling 20k queries per second. If the clients connected directly to the shard with the data, the shard would handle 50-60k queries per second. See mongos1_withlog.ps.

No Logging

The first set ('nolog') is based on v2.2, but with the log() statements in parallel.cpp commented out. This increased mongos' concurrent query volume to 30k queries per second. See mongos1_nolog.ps, and mongod1_nolog.ps.

Short-circuit getShardsForQuery()

The second set ('nogetShardsForQuery') includes the 'nolog' patch, and also immediately returns the first shard:

     void ChunkManager::getShardsForQuery( set<Shard>& shards , const BSONObj& query ) const {
+        shards.insert(_chunkRanges.ranges().begin()->second->getShard());
+        return;

When testing with a query that targets this shard, mongos handles 40-45k queries per second. See mongos1_nogetShardsForQuery.ps, and mongod3_nogetShardsForQuery.ps.