MongoDB Document of Size 300kb taking 8-15s

1. Ensure your operations are not throttled. https://docs.atlas.mongodb.com/reference/free-shared-limitations/

2. Test performance with a different driver (another language), verify you are using most recent driver releases.

3. Test smaller documents to identify whether time is being expended on the server or over the network.

4. Test with mongo shell.

As for an answer, I highly recommend you not to deal with M0 Atlas tier. Or at least choose it wisely, don't choose US-based cluster if you thousand of miles away from States side. Don't understood me wrong. It's a good product. But it depends on your costs.

As for myself, I prefer to deal with MongoDB Community Edition version and deploy it on my VPS/VDS. Of course it doesn't provide you such good web-interface like you have seen in Atlas. And there is no support of Realms functional (stitch), but instead you could design it yourself. And also, every performance issue is depend on you.

As for me, I using MongoDB not for real-time data, but visual snapshots on front-end, and I have no problems with performance.

I mean if I have them, then I deal with them myself, via indexing, increasing VPS CPU/RAM, optimizing queries and so on

Also, one more thing about your problem: «I have documents which have arrays containing 300kb data»

If you have an array field in your schema, and it stores lots of data, especially if it's embedded docs, are you sure that you are using right schema pattern?

You might wanna take a look at this articles at Mongo University about architecture patterns.

Probably it will be much better for you to have a different collection for embedded docs, and request them via aggregation.$lookup when they needed.