Replace boolean OrderBy with DeferBy

orderby consumes the entire input enumerable in 'sorting' the list;
since we don't care about the order besides a yes/no, just manage the
yield order ourselves.

don't bother using this method more than the spot it's in; was a fun
exercise but other spots are set up for easier debugging
(overall flow is where->deferby->yield), could make things easier to
read but meh

PKHeX.Core/Legality/Encounters/Generator/EncounterGenerator.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using static PKHeX.Core.Legal;
 
@@ -420,5 +421,32 @@ namespace PKHeX.Core
             var tr = t.GetOT(pkm.Language);
             return ot == tr;
         }
+
+        /// <summary>
+        /// Filters a sequence of values based on a predicate.
+        /// Any elements that match the predicate are yielded after those that did not match, in the same order they were observed.
+        /// </summary>
+        /// <remarks>
+        /// <see cref="Enumerable"/> OrderBy consumes the entire list when reordering elements, instead of instantly yielding best matches.
+        /// https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,ffb8de6aefac77cc</remarks>
+        /// <typeparam name="T">The type of the elements of source.</typeparam>
+        /// <param name="source">A sequence of values to reorder.</param>
+        /// <param name="predicate">A function to test each element for a condition.</param>
+        /// <returns>An <see cref="IEnumerable{T}"/> that contains elements from the input, with non-deferred results first.</returns>
+        internal static IEnumerable<T> DeferByBoolean<T>(this IEnumerable<T> source, Func<T, bool> predicate)
+        {
+            var deferred = new List<T>();
+            foreach (var x in source)
+            {
+                if (predicate(x))
+                {
+                    deferred.Add(x);
+                    continue;
+                }
+                yield return x;
+            }
+            foreach (var d in deferred)
+                yield return d;
+        }
     }
 }

PKHeX.Core/Legality/Encounters/Generator/EncounterSlotGenerator.cs

@@ -64,7 +64,7 @@ namespace PKHeX.Core
             bool IsHidden = pkm.AbilityNumber == 4; // hidden Ability
             int species = pkm.Species;
 
-            return s.OrderBy(slot => slot.IsDeferred(species, pkm, IsSafariBall, IsSportBall, IsHidden)); // non-deferred first
+            return s.DeferByBoolean(slot => slot.IsDeferred(species, pkm, IsSafariBall, IsSportBall, IsHidden)); // non-deferred first
         }
         public static bool IsDeferred3(this EncounterSlot slot, int currentSpecies, PKM pkm, bool IsSafariBall)
         {