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There could be many reasons for decreased performance in MongoDB, but some of the common ones include:

1. Insufficient hardware resources: If the server running MongoDB doesn't have enough memory, CPU, or disk space, it can slow down performance.

2. Poor indexing: If the queries are not properly indexed or the indexes become fragmented, it can affect query performance.

3. Heavy workloads: If the server is handling too many requests or too much data, it can cause performance issues.

4. Poor query design: If the queries are not optimized or use inefficient operations, it can impact performance.

5. Storage engine issues: If the storage engine is not configured optimally for the workload or there are issues with the storage engine itself, it can affect performance.

To improve performance, it's recommended to:

1. Upgrade hardware resources, such as adding more RAM, CPU or disk space.

2. Optimize indexing and perform regular maintenance to avoid fragmentation.

3. Scale out the system by adding more servers to distribute the workload.

4. Optimize query design or rewrite queries to use more efficient operations.

5. Evaluate and optimize the storage engine configuration or switch to a different storage engine if necessary.