There are several things here:
1 — maybe you should increase the timeout response from the clien call
2 — you should implement a paginate system (it is easy with “limit” and “skip” for mongo and an simple param in the request)
3 — maybe you can read the data with a buffer(like in the example in this article), build the array with the data and then send the response.
I recommend you to start with the second tip, it’s a good practice to paginate elements.
If you control the client too, you can implement the first tip.
And finally , try the third tip