iOS: Intermittently slow network requests - ios

Question

I have an iOS application that has two requests that have large response objects and typically take ~100-200ms to complete when connectivity is good. When connectivity is average these requests seem to get hung-up somewhere ~20% of the time and I've had them take anywhere from 30 to 60s. I'll then restart the app and try the request again seconds later and it will complete in the expected time (100-200 ms)
I've tested the api that the endpoints are querying with other clients (cURL, Postman) and haven't had any issue so I'm fairly certain this has to do with my front-end configuration. I'm using the Alamofire library to handle the requests.
Here's the code for one of the requests:
func login(_ params: [String: String], completion: #escaping (Response<UserResponseSuccess, UserResponseFail>) -> Void) {
Alamofire.request(self.url!, method: .post, parameters: params).responseJSON {
response in
if response.result.isSuccess {
let respJSON: JSON = JSON(response.result.value!)
if response.response?.statusCode == 200 {
// do stuff with json
let resp = UserResponseSuccess()
completion(Response.success(resp))
} else {
logRequestError()
let resp = UserResponseFail()
completion(Response.failure(resp))
}
} else {
logServerError()
let resp = UserResponseFail()
completion(Response.failure(resp))
}
}
}
And here's the request log:
2019-08-17 14:03:16.832462-0600 Debug - foo[17418:3273774] CredStore - performQuery - Error copying matching creds. Error=-25300, query={
class = inet;
"m_Limit" = "m_LimitAll";
ptcl = htps;
"r_Attributes" = 1;
sdmn = "foo.bar.com";
srvr = "foo.bar.com";
sync = syna;
}
$ curl -v \
-X POST \
-H "User-Agent: Debug - foo/1.3 (com.foo.bar.debug; build:4; iOS 12.4.0) Alamofire/4.7.3" \
-H "Content-Type: application/x-www-form-urlencoded; charset=utf-8" \
-H "Accept-Language: en-US;q=1.0" \
-H "Accept-Encoding: gzip;q=1.0, compress;q=0.5" \
-d "email=foo#foo.com&password=bar123" \
"https://rtj.foo.com/users/login"
The requests either go through instantly or don't hit the server. I could probably have these timeout and then retry the request but I don't want to cover up a probable bug.
This also seems to happen more often on a cell network than wifi.
Update: I ran the same query with URLSession instead of Alamofire and received similar results. The error output from the query is:
error:
Error Domain=NSURLErrorDomain Code=-1001 "The request timed out."
UserInfo={NSUnderlyingError=0x28228fe10 {Error
Domain=kCFErrorDomainCFNetwork Code=-1001 "(null)" UserInfo= .
{_kCFStreamErrorCodeKey=60, _kCFStreamErrorDomainKey=1}},
NSErrorFailingURLStringKey=https://foo.bar.com/users/login,
NSErrorFailingURLKey=https://foo.bar.com/users/login,
_kCFStreamErrorDomainKey=1, _kCFStreamErrorCodeKey=60,
NSLocalizedDescription=The request timed out.}
Versioning: Alamofire: 4.7.3, Xcode version: 10.3, Swift version: 5