Is it to be expected to have performance differences when having all documents in one collection or in multiple collections / databases?
Hi Boas,
The answer depends on your use case and dataset. For example, if the frequent queries that your application used can be retrieved from a single collection, then it would be wise to keep the data in a single collection to eliminate an extra query to another collection. See also Data Modelling
Now I’m thinking about wether it would make any difference when I split the data
You may want to check out Sharding. Sharding solves the problem with horizontal scaling. With sharding, you can add more machines to support data growth and the demands of read and write operations. If you already have large dataset to be migrated to MongoDB see Manual Split Chunks in a Sharded Cluster.
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Regards,
Wan.