Using asynchronous methods but Express still returning 503 (NodeJS) - express

Question

I'm developing a web application in NodeJS with an Express back-end. The application is running smoothly except that when a user signs up, a somewhat long operation is called on the back end that involves saving to a database, resizing an image, etc. It takes a few seconds for this process to complete, and during this time anyone else who makes a request to the server will receive a 503 error and will be unable to do anything, whether that's sending or receiving data. I am using asynchronous functions in order to do this whole process. I am using multer to read the file, fs to read and write the file, and Jimp to resize the image. The code is as shown below.
module.exports = function(router) {
router.route('/')
.post(function(req, res, next) {
upload(req, res, function(err) {
var salt = bcrypt.genSaltSync(saltRounds);
var identifier = makeIdentifier(req.body.first_name.trim())
let emailNotifications = 0;
if (req.body.email_notifications === 'true') {
emailNotifications = 1;
}
var signup = {
identifier: identifier,
first_name: toTitleCase(req.body.first_name.trim()),
last_name: req.body.last_name.trim(),
hobby: req.body.hobby,
type: 'user',
email: req.body.email.trim(),
email_notifications: emailNotifications,
password: bcrypt.hashSync(req.body.password, salt)
};
let ascii = /^[ -~]+$/;
for (var propertyName in signup) {
if (!ascii.test(signup[propertyName])) {
res.json({
reason: "invalid-characters"
});
return;
}
}
if (req.file.size > 5000000) {
res.json({
reason: "file-size"
});
return;
}
let emailExists = false;
db.doesEmailExist(req.body.email, function(err, results) {
if (err) {
res.status(500).send("Server error");
return;
} else {
if (results.length > 0) {
res.json({
reason: "email-exists"
});
return;
} else {
fs.readFile(req.file.path, function(err, data) {
let newPath = __dirname + "/profile-pictures/" + identifier + ".png";
fs.writeFile(newPath, data, function(err) {
if (err) {
console.log(err);
} else {
try {
Jimp.read(newPath, (err, file) => {
if (err) throw err;
file
.resize(300, 300) // resize
.quality(60) // set JPEG quality
.write(newPath); // save
});
} catch (error) {
res.json({
reason: "image-properties"
});
return
}
}
});
});
db.signup(signup, function(err, results) {
if (err) {
res.status(500).send("Server error");
return;
} else {
res.json({
success: true
});
return;
}
})
}
}
})
})
});
}
What is causing the server to respond with a 503 error? Any help would be appreciated, thanks!