Very slow post request with Swift's URLSession.shared.dataTask - swift

Question

I'm currently porting an Android app over to iOS and I've noticed a significant decrease in performance for my HTTPS post requests. On Android, using Java HttpsURLConnection objects, post requests would take about 0.5-1.0 seconds on average (for transferring a maximum of about 100 characters). On Swift, using URLSession.shared.dataTask to perform the same post requests takes anywhere from half a second to 15 seconds, sometimes timing out.
One trend I've noticed is that, while running the app, the various requests that are made are either all slow (> 5 seconds) or all faster. The time for requests sporadically changes every time the app is restarted.
Below is my current dataTask code. getResponse and onSuccess are functions for response handling, but I've found that they aren't what's slowing down the requests. From a few "tests" (print statements), it seems like the slow-downs occur as network connections are first being established.
func execute() {
guard let url = URL(string: "https://website.com") else {
print("ERROR > INVALID URL")
return
}
var request = URLRequest(url: url)
request.httpMethod = "POST"
let postDataString = "data"
guard let requestBody: Data = postDataString.data(using: String.Encoding.utf8) else {
print("ERROR > FAILED TO ENCODE POST STRING")
return
}
request.httpBody = requestBody
let task = URLSession.shared.dataTask(with: request) { (data: Data?, response: URLResponse?, error: Error?) in
if let unwrappedError = error {
print("ERROR > FAILED TO CONNECT TO SERVER: \(error)")
return
}
guard let unwrappedData = data else {
print("ERROR > FAILED TO GET DATA")
return
}
guard let dataString = String(data: data, encoding: String.Encoding.utf8)?.replacingOccurrences(of: "+", with: "%2B") else {
print("ERROR > FAILED TO DECODE RESPONSE TO STRING")
return
}
let dataStringLines: [String] = dataString.components(separatedBy: "\n")
guard let unwrappedGetResponse = self.getResponse else {
print("ERROR > getResponse FUNCTION UNINITIALIZED")
return
}
let success: Bool = unwrappedGetResponse(dataStringLines)
if (success) {
guard let unwrappedOnSuccess = self.onSuccess else {
print("ERROR > onSuccess FUNCTION UNINITIALIZED")
return
}
DispatchQueue.main.async {
unwrappedOnSuccess()
}
}
else {
print("ERROR > BAD SERVER RESPONSE")
}
}
task.resume()
}
Some additional information: I'm testing the code with an iPhone 5S running iOS 12.4.5. The app is targeting iOS 10.0.
Any idea on what could cause such inconsistent performance?