Why does it take a long time to get a response from the retrofit in some cases?

Retrofit @get request sometimes It takes a long time to get the answer (10-15 sec.). I am only getting string data. on average 1-2 kb.

No problem when I send request from postman

What do you think is the problem?

RetrofitService.class;

 private val okHttpClient = OkHttpClient().newBuilder()
        .connectTimeout(20, TimeUnit.SECONDS)
        .readTimeout(20, TimeUnit.SECONDS)
        .writeTimeout(20, TimeUnit.SECONDS)
        .build()

private val call = Retrofit.Builder()
    .baseUrl(Constants.LINK)
    .client(okHttpClient)
    .addConverterFactory(GsonConverterFactory.create())
    .addCallAdapterFactory(RxJava3CallAdapterFactory.create())
    .build().create(ApiEndpointRXJava::class.java)


fun getRetrofit(query: String, auth: String, queryList: List<String>): Single<List<ListModel>> {
    return  call.getRetrofit(query, auth, queryList)
}

Interface;

    @GET(".../.../...")
    fun getRetrofit(
        @Query("query") query: String,
        @Header("Authorization") authorization: String,
        @Query("queryList") queryList: List<String>
    ): Single<List<ListModel>>

ViewModel.class;

fun getRetrofit(query: String, auth: String, queryList: List<String>) {
    errorData.value = false

    disposable.add(
        retrofit
            .getRetrofit(query, auth, queryList)
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribeWith(object : DisposableSingleObserver<List<ListModel>>() {
                override fun onSuccess(t: List<ListModel>) {
                    v.value = t
                }

                override fun onError(e: Throwable) {
                    e.value = true
                }
            })
    )
}