Pools renamed to FixedCapacityPools. Other fixed capacity collections added. Queues, concurrent queues and standard stacks replaced with fixed capacity stacks. MethodImplOptions.AggressiveInlining and .AggressiveOptimization added to simple methods and getters / setters. Variables optimized in few places.

PerfectNumbers.Core/UlongArrayPool.cs

@@ -0,0 +1,40 @@
+using System.Runtime.CompilerServices;
+
+namespace PerfectNumbers.Core
+{
+	public struct UlongArrayPool(int poolCapacity)
+	{
+        private readonly ulong[][] _arrays = new ulong[poolCapacity][];
+        private int _count = 0;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public ulong[] Rent(int length)
+        {
+            int available = _count;
+            if (available == 0)
+            {
+                return new ulong[length];
+            }
+
+            ulong[][] arrays = _arrays;
+            available--;
+            _count = available;
+
+            ulong[] candidate = arrays[available];
+			return candidate.Length >= length ? candidate : new ulong[length];
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
+        public void Return(ulong[] array)
+        {
+            ulong[][] arrays = _arrays;
+            int available = _count;
+
+            if (available < arrays.Length)
+            {
+				arrays[available] = array;
+				_count = available + 1;
+            }
+        }
+    }
+}