Understanding RAM and CPU requirements

The minimum CPU requirements for MongoDB are quite light. The MongoDB team recommends that each mongod instance be given access to two cores in a cloud environment, or a multi-core CPU when running on a physical server. The WiredTiger engine is multithreaded and is able to take advantage of multiple cores. Generally speaking, the number of CPU cores available improves throughput and performance. When the number of concurrent users increases, however, if operations occupy all available cores, a drastic downturn in throughput and performance is observed.

Have a look at the WiredTiger section in the MongoDB Administration - Production Notes (https://docs.mongodb.com/manual/administration/production-notes/#hardware-considerations) for more information.

As of MongoDB 3.4, the WiredTiger internal cache will default either to 50% of RAM (1 GB), or 256 megabytes (MB), whichever is larger.

Accordingly, you need to make sure that you have enough RAM available...