You can change the block size used in a Btree to affect the height or number of levels in a Btree collection.
Theory and test show that minimizing the height of a Btree reduces both the search and update time for Btree operations.
The improvements occur in the following areas.
Reduced CPU time for Btree operations when Btree is cache-resident
Improvements in overall disk and network retrieval times, by reducing the number of fetches, mitigating the effects of transfer latency
However, you should consider the following potential negative effects.
Increase in transaction journal sizes, because the entire collection block images are audited
Increased object cache utilization for certain collection access patterns