IIS slows down after many requests

[mwjhartogs]

I'm running into a performance issues with an entity framework linq query.
DLL versions:
npgsql 4.0.8
EntityFramework6.Npgsql 3.2.1.1
DotNet 4.6.1
As addon I use Entity Framework Plus (Z.EntityFramework.Plus.QueryIncludeFilter.EF6)

I'm running a big query in C# (web api). This request will be called many times per hour (>2000 times) via a web api Visual studio 2015 project. After almost every call the execution time will be higher. I'm closing the database contexts in a right way (I Believe). I really don't have a clue why the application pool of IIS getting slower and slower. Recycling the application pool will bring back a good performance. Same happens when restarting IIS express in Visual studio (debug mode).

My code;

Stopwatch stopWatch = new Stopwatch();

  //It's a multi tentant application. So every customer has it's own connection string. 
                // For this test I only run with the same 
                using (var context = new appEntities("app" + organization))
                {
                    stopWatch.Start();
                    context.Configuration.EnsureTransactionsForFunctionsAndCommands = false;
                    context.Configuration.ProxyCreationEnabled = false;
                    context.Configuration.LazyLoadingEnabled = false;
                    context.Configuration.AutoDetectChangesEnabled = false;
                    context.Configuration.ValidateOnSaveEnabled = false;
                    beeldkwaliteit_formulieren form =
                                context.beeldkwaliteit_formulieren
                                       .Where(x => x.id == 34)
                       .IncludeFilter(x => x.inspectie_categorie
                                       .Where(y => y.id_typebeeldkwaliteit == id_typebeeldkwaliteit))
                       .IncludeFilter(x => x.inspectie_categorie
                           .Where(i => i.id_form == 34).Select(z => z.inspectie_sub_categorie))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                           .Where(y => y.id_form == 34))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                                .Select(y => y.beeldkwaliteit_insp_form_values
                                        .Where(z => z.id_insp == id_inspectie)))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                                 .Select(y => y.beeldkwaliteit_fields_activated_by))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                                 .Select(y => y.beeldkwaliteit_waardenlijst)
                                    .Where(z => z.id_typebeeldkwaliteit == id_typebeeldkwaliteit))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                              .Select(y => y.beeldkwaliteit_waardenlijst.beeldkwaliteit_waardenlijst_values))
                       .IncludeFilter(x => x.beeldkwaliteit_insp_form_fields
                            .Select(y => y.beeldkwaliteit_waardenlijst.beeldkwaliteit_waardenlijst_values
                            .Select(z => z.beeldkwaliteit_waardenlijst_values_linked_values)))
                       .FirstOrDefault();
                    stopWatch.Stop();
                    Debug.WriteLine("SW:" + stopWatch.ElapsedMilliseconds);
                }

My connect string (every customer has one)

<add name="app_customer" connectionString="metadata=res://*/Models.****DBModel.csdl|res://*/Models.appNameDBModel.ssdl|res://*/Models.*****DBModel.msl;provider=Npgsql;provider connection string=&quot;Host=www.******.eu;Database=dev_*****;Username=******;Password=*****;Search Path=public&quot;" providerName="System.Data.EntityClient" />

The stopwatch times:
Start: SW:1652
SW:112
SW:106
SW:109
SW:168
SW:116
-- Skip 30
SW:117
-- Skip 30
SW:167
-- Skip 30
SW:258
-- Skip 30
SW:271
-- skip 30
SW:303
-- skip 30
SW:376
-- skip 30
SW:578
-- skip 30
SW:571
-- skip 30
SW:617
-- skip 30
SW:655