mongodb connection timeout when callback function after query is slow - node.js

Question

I am working on the back-end of a web application. Server-side, node is used along with express and mongo (via mongoose).
The client can make a post request that does the following:
load the file location of some data from mongo
load the file and do some heavy work with it
send the results to the client
This is done by a function that looks like this:
function (req, res, next) {
// read relevant info from req object, e.g.
var id = req.body.id;
mySchema.findById(id, (err, doc) => {
fs.readFile(doc.location, function (err, data) {
// do some heavy work with data
res.statusCode = 200;
res.setHeader('Content-Type', 'application/json');
var bodyContent = [];
// fill bodyContent
res.write(JSON.stringify(bodyContent));
res.end();
});
});
};
This works fine for small files, but when the files become large and the computations of the "heavy work" take longer to execute, mongo throws connection timeout errors:
Error: connection timeout
at Db.<anonymous> (<project path>/node_modules/mongoose/lib/drivers/node-mongodb-native/connection.js:169:17)
at emitTwo (events.js:106:13)
at Db.emit (events.js:191:7)
at Server.listener (<project path>/node_modules/mongoose/node_modules/mongodb/lib/db.js:1798:14)
at emitOne (events.js:96:13)
at Server.emit (events.js:188:7)
at Server.<anonymous> (<project path>/node_modules/mongoose/node_modules/mongodb/lib/server.js:274:14)
at emitOne (events.js:96:13)
at Server.emit (events.js:188:7)
at Pool.<anonymous> (<project path>/node_modules/mongoose/node_modules/mongodb-core/lib/topologies/server.js:335:12)
at emitOne (events.js:96:13)
at Pool.emit (events.js:188:7)
at Connection.<anonymous> (<project path>/node_modules/mongoose/node_modules/mongodb-core/lib/connection/pool.js:270:12)
at Connection.g (events.js:291:16)
at emitTwo (events.js:106:13)
at Connection.emit (events.js:191:7)
at Socket.<anonymous> (<project path>/node_modules/mongoose/node_modules/mongodb-core/lib/connection/connection.js:185:10)
at Socket.g (events.js:291:16)
at emitNone (events.js:86:13)
at Socket.emit (events.js:185:7)
at Socket._onTimeout (net.js:339:8)
at ontimeout (timers.js:365:14)
at tryOnTimeout (timers.js:237:5)
at Timer.listOnTimeout (timers.js:207:5)
I have tried to increase the time before timeout as described in other posts related to mongo timeout issues and as described here, but to no avail. What is the best approach to solve such an issue?
Update:
Looking into the problem more closely, a possible solution seems to be disconnecting Mongo at the start of the callback, e.g. via mongoose.connection.close() and re-connect just before the res.end() line with something like mongoose.connect(config). However, I have not found a way to do this such that it functions when multiple requests are coming in. Any thoughts on this?